Yes, It has being part of a number of discussions in the past.
- Allowing us to edit with alternative previews visible above would resemble many existing wiki out there, like wikipedia/wikimedia implementations and help people less familiar with mark-up/down rather than WYSIWYG.
- There may even be some solutions out there.
Not only can it be, somehow in a couple of wikis of mine it is doing it without me intending it to do so, and I am having trouble reversing it. Recently I believe there was some changes to introduce a more flexible layout.
Another approach I have, that is in progress, is making the existing previews or additional ones both available in the view and edit templates above or below the body. I will share more on review of my own notes/work, I have touched this idea so many times I have notes everywhere 
To find a quick fix
Currently the horizontal layout editor to left of preview is achieved by them adding up to less than 100% of the width. If you increase the width of the preview it falls below.
- I am keen to see something similar to the “sidebar resizer” to change the ratio between the two edit and preview
- But we could also change it from horizontal to vertical with a right hand drag on both, and a method to swap the order.
A somewhat related discussion for the codemirror 6 plugin is happening here Plugin: Another way to preview tiddler on edit