Ah, I just found another post asking for basically the same thing...indeed, this is exactly what I would like to do. The controls for cropping are disabled in each input's position settings with multiview...I hope whatever prevents that from working can be fixed :)

Personally, I'm not looking to stretch. I want to have 2 cameras available that I can switch to and go fullscreen on at will, but also have a multiview set up with those two inputs where the top quarter and bottom quarter of each video is cropped out, and then stack them on top of each other (or chop off the sides and put them side by side) so that each video is not warped, but it still takes up the full viewing area.

I was experimenting with it last night, and I was able to come up with a hack to get it to work, but it's not pretty. I had to add 2 copies of each camera as inputs, and crop one of each at the source before adding them to an overlay.

The result of what I'm looking for looks something like this: http://i.imgur.com/j9gEWJp.jpg

I just wish I didn't have to add two of each camera just to do this, which would be possible if I could crop overlays in multiviews.