Ok...it looks like you just have a problem with mesh flow. To change it, use grid-auto-flow:column; That's it, the grid's flow will change from rows to columns. You can change grid-template-rows as needed.

What you need to do in html is:

   
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and CSS:

.parent{
     display:grid;
     grid-template-columns: 1fr 1fr;
     grid-auto-flow: column;
     grid-template-rows:1fr 1fr 1fr;
 }