Well, I'm not sure if this is a bug or not... In one of my games, I met a neutral enemy party composed of a bandit and a wolf. When selecting the party, two icon appears in the left side of the interface (as when you select one of your own parties). Clicking in the small arrow that appears in these icons, you can separate the enemy party (as you can do with your own units), so in the next turn, both enemies sepparated and followed different ways, and I was able to combat them in different turns. If I fighted them together, they'll probably end up killing my soldier, but, in this way, I killed both of them.
In case the units weren't a party, but two sepparate enemies, maybe they shoul not be allowed to ocuppy the sime tile.
I will try to reproduce it and post some pictures next time I play.