Ok so what if there was a setting that allowed players to make agreements with each other so that multiple players could occupy the same territory and/or allowed other players to pass through your territory that you occupy?
A rights of passage would be easier to implement I think. Dual occupancy of a territory might take a little more work. Maybe the disadvantage that would be included in this is that nobody would get credit for holding this territory (but it would not hinder someone from collecting the continent bonus if they hold the rest of it). The limitations to duel occupancy would be that you could only do it in one territory per continent.