We provide a handful of layouts, but you can specify custom layouts (including a side-by-side 3D+slice view) by an XML string. This example in the script repository does almost exactly what you describe. You can change vertical to horizontal to have side-by-side instead of over-under.
The idea of making layout customization easier for clinicians or make layouts more dynamic comes up time-to-time (see the issues below) but overall there is not too many upvotes for these features:
- Allow user to specify favorite views · Issue #5497 · Slicer/Slicer · GitHub
- arbitrary screen layout grids · Issue #2384 · Slicer/Slicer · GitHub
- layout modification for better editing · Issue #1359 · Slicer/Slicer · GitHub
You can upvote the issues that you find the most relevant. You may also find other feature request that are closer to what you want to achieve, or you can add a new feature request if you feel that none of them describe what you want.