Current behavior:

It's not possible to customize the layout of panes in the Editor for Web.

Workaround:

Download the Bilingual MXLIFF and work in Editor for Desktop where you can customize Editor's layout.

Requested behavior: 

It's not possible to customize the layout of panes in the Editor for Web.

Use case: 

To make the work easier for reviewers. If they are able to see the CAT tab (with TM matches) and the LQA tab (with LQA errors) at the same time, this will allow them to see the TM matches and TB hits, while working on the LQA error logging. It will also allow them to easily copy/paste the original translation to the LQA comment.