I'd keep Osgiliath 2 cities, but give them battle maps that overlap. For a rough idea of how this would look check out this mock up:
The green area would be the defender's deployment zone while the red would be the attackers.
The benefit of doing it this way is that it would still feel like you were fighting in one big city but it would allow Gondor to hold onto West Osgiliath even after the east side falls. Of course, I don't even know if something like this would work given the tools being used, i still think it'd be cool though.