I am jumping on this thread way late (and must confess, I have skimmed and not read every post) so if this is redundant I apologize. But I often thought, that, once the boundaries were settled, that all the bishops in the common diocese would serve as a "junta" with a rotating head bishop every year. This arrangement would continue until they all passed away, or voluntarily retired. Then a new single bishop would be consecrated.
This would be a way that no bishop would have to "step down" or retire unwillingly.
It's probably entirely non-canonical, impractical, etc. but it is the sort of short-tem compromise that might get people over the hump of unification