To me the split view experience sounds pretty much as good between both devices. He doesn’t like the lack of 3-column views in some apps on the 11” iPad Pro, but I rarely use 3-column so I’d expect I don’t miss it at all.
I’ve thought about getting an 11” iPad Pro in addition to my 12.9”, but I’d use that smaller tablet for different things. Like dealing with email while my 12.9” is rendering video or carrying around the house and working remotely.
The 12.9” iPad Pro is a great device, but I do think that sometimes it’s size limits how awesome it is. That said, I’d totally be in for a 15” iPad Pro that was at my desk and an 11” that was my mobile device. It seems a bit crazy to say that.