On iOS 11 for iPad, when you're in the "multitasking/Mission Control-like" screen, can you drag one of the "full screen" app tiles onto another full screen app tile and join them as a "split screen" tile? [Does that even make sense?] You can do this with two full screen apps in macOS's Mission Control.
On a side note, I think "Mission Control" is a horrible name.
You can't, yet it's the first thing I tried because it's the most intuitive way to do it. I hope that it will be added, though I can see a up challenge on doing "the right thing" when one or more of the spaces you merge is already in split view (my first thought is that it'd be nice to be able to merge/unmerge an app directly in that view)