Expanding cells in CSS grid layout

Answer 1

This is something better suited to flexbox but if your structure is as simplistic as you indicate the only-child can be used here.

.main {
  display: grid;
  grid-template-columns: 2fr 1fr;
  margin-bottom: 1em;
}


/* The following is not essential - for decoration purposes only */

.left {
  background-color: green;
  height: 25vh;
}

.left:only-child {
  grid-column: 1 / -1;
}

.right {
  background-color: orange;
}

<div class="main">
  <div class="left">Left</div>
  <div class="right">Right</div>
</div>

<div class="main">
  <div class="left">Left</div>
</div>

Answer 2

You can rely on implicit grid creation:

.main {
  display: grid;
  grid-template-columns: 2fr;
  grid-auto-columns:1fr; /* this will trigger when you add the "right" element */
  grid-auto-flow:column;
  margin:5px;
}


.left {
  background-color: green;
}

.right {
  background-color: orange;
}

<div class="main">
  <div class="left">Left</div>
  <div class="right">right</div>
</div>

<div class="main">
  <div class="left">Left</div>
</div>

It does also work if you want to remove left:

.main {
  display: grid;
  grid-template-columns: 2fr;
  grid-auto-columns:1fr; /* this will trigger when you add the "left" element */
  grid-auto-flow:column;
  margin:5px;
}


.left {
  background-color: green;
  grid-column-end:1; /* added this */
}

.right {
  background-color: orange;
}

<div class="main">
  <div class="left">Left</div>
  <div class="right">right</div>
</div>

<div class="main">
  <div class="right">right</div>
</div>