I believe the solution we settled on RE: XP & Co-MCing was
Josh selected MC's Choice for Judson, Tablesaw and Adam. Adam selected MC's choice for Josh.
The remaining two players selected Player's Choice for each player, debating between them and coming to consensus.

Though I felt we were fairly successful in our co-MCing experiment, we did wish for more guidance on how to make it work. We all tend to think and play in terms of scenes, so we had some advantage in that we were setting those scenes with specific characters and a player who didn't have a present PC could become MC.  But as Josh has mentioned, we're all fairly experienced GMs and familiar with PbtA. Especially for new and unfamiliar players, I think this co-MC concept could be very intimidating without more guidance. I assume that guidance will be in the final product, but I would urge some extensive suggestions of "how" for those who want to try it.