960 Grid System nested grid units

I'm trying to use nested grids with the 960 grid system, which according to the 960.gs website should work fine if the first child is assigned the alpha class and the last child assigned the omega class. However, I've found that even with these classes, the first child grid appears in the correct position, while any further children appear slightly offset. An example of this can be seen here:

HTML:

<html>
<head>
    <title>Test</title>
</head>
<body>
    <div class="container_12" style="background:#454545">
        <div class="grid_12" id="wrapper">
            <div class="grid_12 alpha row highlight">
            1
            </div>
            <div class="grid_12 omega row highlight">
            2
            </div>
        </div>
    </div>
</body>
</html>

CSS:

/*
  960 Grid System ~ Core CSS.
  Learn more ~ http://960.gs/

  Licensed under GPL and MIT.
*/

/*
  Forces backgrounds to span full width,
  even if there is horizontal scrolling.
  Increase this if your layout is wider.

  Note: IE6 works fine without this fix.
*/

body {
  min-width: 960px;
}

/* `Container
----------------------------------------------------------------------------------------------------*/

.container_12,
.container_16 {
  margin-left: auto;
  margin-right: auto;
  width: 960px;
}

/* `Grid >> Global
----------------------------------------------------------------------------------------------------*/

.grid_1,
.grid_2,
.grid_3,
.grid_4,
.grid_5,
.grid_6,
.grid_7,
.grid_8,
.grid_9,
.grid_10,
.grid_11,
.grid_12,
.grid_13,
.grid_14,
.grid_15,
.grid_16 {
  display: inline;
  float: left;
  margin-left: 10px;
  margin-right: 10px;
}

.push_1, .pull_1,
.push_2, .pull_2,
.push_3, .pull_3,
.push_4, .pull_4,
.push_5, .pull_5,
.push_6, .pull_6,
.push_7, .pull_7,
.push_8, .pull_8,
.push_9, .pull_9,
.push_10, .pull_10,
.push_11, .pull_11,
.push_12, .pull_12,
.push_13, .pull_13,
.push_14, .pull_14,
.push_15, .pull_15 {
  position: relative;
}

.container_12 .grid_3,
.container_16 .grid_4 {
  width: 220px;
}

.container_12 .grid_6,
.container_16 .grid_8 {
  width: 460px;
}

.container_12 .grid_9,
.container_16 .grid_12 {
  width: 700px;
}

.container_12 .grid_12,
.container_16 .grid_16 {
  width: 940px;
}

/* `Grid >> Children (Alpha ~ First, Omega ~ Last)
----------------------------------------------------------------------------------------------------*/

.alpha {
  margin-left: 0;
}

.omega {
  margin-right: 0;
}

/* `Grid >> 12 Columns
----------------------------------------------------------------------------------------------------*/

.container_12 .grid_1 {
  width: 60px;
}

.container_12 .grid_2 {
  width: 140px;
}

.container_12 .grid_4 {
  width: 300px;
}

.container_12 .grid_5 {
  width: 380px;
}

.container_12 .grid_7 {
  width: 540px;
}

.container_12 .grid_8 {
  width: 620px;
}

.container_12 .grid_10 {
  width: 780px;
}

.container_12 .grid_11 {
  width: 860px;
}

/* `Grid >> 16 Columns
----------------------------------------------------------------------------------------------------*/

.container_16 .grid_1 {
  width: 40px;
}

.container_16 .grid_2 {
  width: 100px;
}

.container_16 .grid_3 {
  width: 160px;
}

.container_16 .grid_5 {
  width: 280px;
}

.container_16 .grid_6 {
  width: 340px;
}

.container_16 .grid_7 {
  width: 400px;
}

.container_16 .grid_9 {
  width: 520px;
}

.container_16 .grid_10 {
  width: 580px;
}

.container_16 .grid_11 {
  width: 640px;
}

.container_16 .grid_13 {
  width: 760px;
}

.container_16 .grid_14 {
  width: 820px;
}

.container_16 .grid_15 {
  width: 880px;
}

/* `Prefix Extra Space >> Global
----------------------------------------------------------------------------------------------------*/

.container_12 .prefix_3,
.container_16 .prefix_4 {
  padding-left: 240px;
}

.container_12 .prefix_6,
.container_16 .prefix_8 {
  padding-left: 480px;
}

.container_12 .prefix_9,
.container_16 .prefix_12 {
  padding-left: 720px;
}

/* `Prefix Extra Space >> 12 Columns
----------------------------------------------------------------------------------------------------*/

.container_12 .prefix_1 {
  padding-left: 80px;
}

.container_12 .prefix_2 {
  padding-left: 160px;
}

.container_12 .prefix_4 {
  padding-left: 320px;
}

.container_12 .prefix_5 {
  padding-left: 400px;
}

.container_12 .prefix_7 {
  padding-left: 560px;
}

.container_12 .prefix_8 {
  padding-left: 640px;
}

.container_12 .prefix_10 {
  padding-left: 800px;
}

.container_12 .prefix_11 {
  padding-left: 880px;
}

/* `Prefix Extra Space >> 16 Columns
----------------------------------------------------------------------------------------------------*/

.container_16 .prefix_1 {
  padding-left: 60px;
}

.container_16 .prefix_2 {
  padding-left: 120px;
}

.container_16 .prefix_3 {
  padding-left: 180px;
}

.container_16 .prefix_5 {
  padding-left: 300px;
}

.container_16 .prefix_6 {
  padding-left: 360px;
}

.container_16 .prefix_7 {
  padding-left: 420px;
}

.container_16 .prefix_9 {
  padding-left: 540px;
}

.container_16 .prefix_10 {
  padding-left: 600px;
}

.container_16 .prefix_11 {
  padding-left: 660px;
}

.container_16 .prefix_13 {
  padding-left: 780px;
}

.container_16 .prefix_14 {
  padding-left: 840px;
}

.container_16 .prefix_15 {
  padding-left: 900px;
}

/* `Suffix Extra Space >> Global
----------------------------------------------------------------------------------------------------*/

.container_12 .suffix_3,
.container_16 .suffix_4 {
  padding-right: 240px;
}

.container_12 .suffix_6,
.container_16 .suffix_8 {
  padding-right: 480px;
}

.container_12 .suffix_9,
.container_16 .suffix_12 {
  padding-right: 720px;
}

/* `Suffix Extra Space >> 12 Columns
----------------------------------------------------------------------------------------------------*/

.container_12 .suffix_1 {
  padding-right: 80px;
}

.container_12 .suffix_2 {
  padding-right: 160px;
}

.container_12 .suffix_4 {
  padding-right: 320px;
}

.container_12 .suffix_5 {
  padding-right: 400px;
}

.container_12 .suffix_7 {
  padding-right: 560px;
}

.container_12 .suffix_8 {
  padding-right: 640px;
}

.container_12 .suffix_10 {
  padding-right: 800px;
}

.container_12 .suffix_11 {
  padding-right: 880px;
}

/* `Suffix Extra Space >> 16 Columns
----------------------------------------------------------------------------------------------------*/

.container_16 .suffix_1 {
  padding-right: 60px;
}

.container_16 .suffix_2 {
  padding-right: 120px;
}

.container_16 .suffix_3 {
  padding-right: 180px;
}

.container_16 .suffix_5 {
  padding-right: 300px;
}

.container_16 .suffix_6 {
  padding-right: 360px;
}

.container_16 .suffix_7 {
  padding-right: 420px;
}

.container_16 .suffix_9 {
  padding-right: 540px;
}

.container_16 .suffix_10 {
  padding-right: 600px;
}

.container_16 .suffix_11 {
  padding-right: 660px;
}

.container_16 .suffix_13 {
  padding-right: 780px;
}

.container_16 .suffix_14 {
  padding-right: 840px;
}

.container_16 .suffix_15 {
  padding-right: 900px;
}

/* `Push Space >> Global
----------------------------------------------------------------------------------------------------*/

.container_12 .push_3,
.container_16 .push_4 {
  left: 240px;
}

.container_12 .push_6,
.container_16 .push_8 {
  left: 480px;
}

.container_12 .push_9,
.container_16 .push_12 {
  left: 720px;
}

/* `Push Space >> 12 Columns
----------------------------------------------------------------------------------------------------*/

.container_12 .push_1 {
  left: 80px;
}

.container_12 .push_2 {
  left: 160px;
}

.container_12 .push_4 {
  left: 320px;
}

.container_12 .push_5 {
  left: 400px;
}

.container_12 .push_7 {
  left: 560px;
}

.container_12 .push_8 {
  left: 640px;
}

.container_12 .push_10 {
  left: 800px;
}

.container_12 .push_11 {
  left: 880px;
}

/* `Push Space >> 16 Columns
----------------------------------------------------------------------------------------------------*/

.container_16 .push_1 {
  left: 60px;
}

.container_16 .push_2 {
  left: 120px;
}

.container_16 .push_3 {
  left: 180px;
}

.container_16 .push_5 {
  left: 300px;
}

.container_16 .push_6 {
  left: 360px;
}

.container_16 .push_7 {
  left: 420px;
}

.container_16 .push_9 {
  left: 540px;
}

.container_16 .push_10 {
  left: 600px;
}

.container_16 .push_11 {
  left: 660px;
}

.container_16 .push_13 {
  left: 780px;
}

.container_16 .push_14 {
  left: 840px;
}

.container_16 .push_15 {
  left: 900px;
}

/* `Pull Space >> Global
----------------------------------------------------------------------------------------------------*/

.container_12 .pull_3,
.container_16 .pull_4 {
  left: -240px;
}

.container_12 .pull_6,
.container_16 .pull_8 {
  left: -480px;
}

.container_12 .pull_9,
.container_16 .pull_12 {
  left: -720px;
}

/* `Pull Space >> 12 Columns
----------------------------------------------------------------------------------------------------*/

.container_12 .pull_1 {
  left: -80px;
}

.container_12 .pull_2 {
  left: -160px;
}

.container_12 .pull_4 {
  left: -320px;
}

.container_12 .pull_5 {
  left: -400px;
}

.container_12 .pull_7 {
  left: -560px;
}

.container_12 .pull_8 {
  left: -640px;
}

.container_12 .pull_10 {
  left: -800px;
}

.container_12 .pull_11 {
  left: -880px;
}

/* `Pull Space >> 16 Columns
----------------------------------------------------------------------------------------------------*/

.container_16 .pull_1 {
  left: -60px;
}

.container_16 .pull_2 {
  left: -120px;
}

.container_16 .pull_3 {
  left: -180px;
}

.container_16 .pull_5 {
  left: -300px;
}

.container_16 .pull_6 {
  left: -360px;
}

.container_16 .pull_7 {
  left: -420px;
}

.container_16 .pull_9 {
  left: -540px;
}

.container_16 .pull_10 {
  left: -600px;
}

.container_16 .pull_11 {
  left: -660px;
}

.container_16 .pull_13 {
  left: -780px;
}

.container_16 .pull_14 {
  left: -840px;
}

.container_16 .pull_15 {
  left: -900px;
}

/* `Clear Floated Elements
----------------------------------------------------------------------------------------------------*/

/* http://sonspring.com/journal/clearing-floats */

.clear {
  clear: both;
  display: block;
  overflow: hidden;
  visibility: hidden;
  width: 0;
  height: 0;
}

/* http://www.yuiblog.com/blog/2010/09/27/clearfix-reloaded-overflowhidden-demystified */

.clearfix:before,
.clearfix:after,
.container_12:before,
.container_12:after,
.container_16:before,
.container_16:after {
  content: '.';
  display: block;
  overflow: hidden;
  visibility: hidden;
  font-size: 0;
  line-height: 0;
  width: 0;
  height: 0;
}

.clearfix:after,
.container_12:after,
.container_16:after {
  clear: both;
}

/*
  The following zoom:1 rule is specifically for IE6 + IE7.
  Move to separate stylesheet if invalid CSS is a problem.
*/

.clearfix,
.container_12,
.container_16 {
  zoom: 1;
}    

html,body {
        margin:0;
    }

    #wrapper {

    }

    .row {
        margin-bottom:10px;
    }

    .highlight {
        background:yellow;
    }

Does anyone have a fix to this issue?

Solution

First of all, you are misusing grid_12 where you should use container_12 in the wrapper.

Moreover alpha and omega classes are designed for aligned cells, that means that two grid_12 divs cannot be aligned inside a container_12 div (that would be 24 columns, and this does not fit)

Lastly, as I understood, it seems that you want to turn that into two aligned grid_6 div

Hope this helps !