The Stephens would definitely be turned on the idea. I do think it would be weird for the Jons - they've each been built, slowly but surely, into a competent sub/Dom by interacting with their respective partners, but neither of them has experience at the opposite role. So that would either get incredibly awkward or be a fun change of pace.

I can't see the sparks flying between Jon and Jon, but the three-way or four-way combinations could get fun. (C!Stephen getting dommed by two Jons at once...mmm.)

And, yeah, alt!Jon is not so alt!, except in the sense that his interactions with his Stephen have shaped him differently. Jon is Zen enough to be very consistent, no matter what universe you're in =)