How can I have shared borders on Bootstrap columns?

Question

I am trying to create a Bootstrap container that has two columns. On larger screens, the columns should be side by side horizontally, on smaller screens they should be stacked vertically. There should be a 1px border around the columns with rounded corners. As per the mockup images below:

two columns on large screens

two columns on small screens

How can I do this? Notice where the columns touch on both large and small screens, the borders are still only a total of 1px and are not rounded.

Here's where I'm at right now:

.custom-left-column {
  border: 1px solid #ccc;
  background-color: #fff;
  padding: 15px;
  border-top-left-radius: 10px;
  border-bottom-left-radius: 10px;
}

.custom-right-column {
  border: 1px solid #ccc;
  background-color: #e9ecef;
  padding: 15px;
  border-top-right-radius: 10px;
  border-bottom-right-radius: 10px;
}

<link
  rel="stylesheet"
  href="https://stackpath.bootstrapcdn.com/bootstrap/5.0.0/css/bootstrap.min.css"
/>
<div class="container-fluid mx-auto" style="max-width: 70%">
  <div class="row">
    <!-- Left Column-->
    <div
      class="col-lg-8 d-flex align-items-center mx-auto custom-left-column"
    >
      <p>Left Column</p>
    </div>

    <!-- Right Column -->
    <div
      class="col-lg-4 d-flex align-items-center mx-auto custom-right-column"
    >
      <p>Right Column</p>
    </div>
  </div>
</div>

Answer 1

Try this code:

.custom-left-column, .custom-right-column {
  border: 1px solid #ccc;
  padding: 15px;
  text-align: center;
}

.custom-left-column {
  background-color: #fff;
  border-top-left-radius: 10px;
  border-bottom-left-radius: 10px;
}

.custom-right-column {
  background-color: #e9ecef;
  border-top-right-radius: 10px;
  border-bottom-right-radius: 10px;
}

@media screen and (max-width:768px) {
  .custom-left-column {
    border-radius: 0;
    border-top-left-radius: 10px;
    border-top-right-radius: 10px;
  }

  .custom-right-column {
    border-radius: 0;
    border-bottom-left-radius: 10px;
    border-bottom-right-radius: 10px;
  }
}

<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-T3c6CoIi6uLrA9TneNEoa7RxnatzjcDSCmG1MXxSR1GAsXEV/Dwwykc2MPK8M2HN" crossorigin="anonymous">

 
<div class="row m-2">
<!-- Left Column-->
  <div class="col-md-9 align-items-center custom-left-column">
    <p>Left Column</p>
  </div>

  <!-- Right Column -->
  <div class="col-md-3 align-items-center custom-right-column">
    <p>Right Column</p>
  </div>
</div>