I have 2 tablix one Table and one Matrix that are lined up side by side. They have relational data that I can't combine in a single query. They line up perfectly when exported and appear to line up correctly inside the original report as well. However, when it goes to load a new "Page" in Preview it loads an extra row for the matrix as compared to the table which makes the rest of the values off by 1 for each page that gets loaded.
Is there a good way to make sure they load the same number of rows on each page?
I have 2 suggestions. You probably already tried the first, but just want to make sure it's covered.
Check the properties to make sure the location from the top is exactly the same. Also use a whole number, not some random fraction because it can have rounding issues.
Add a rectangle to the report. Drag both tables onto the rectangle. Having multiple objects inside a rectangle helps to hold them together in position when the report renders.