@Lord Reliant: Lairs spawn new groups of enemies periodically. A new group might go wander off, or it might stay in the tile. If it stays, you can generally tell by the fact they get a special icon over them. Clicking again on such a unit will show you the unit underneath the top one.
I don't think two different monster armies should be able to occupy the same tile. Why wouldn't they just become one larger unit? Otherwise it's like:
"This should be easy, I'll destroy this lone wolf army."
[after battle]
"Yay! Uh oh. Under that lone wolf was an army of 5 more! And now I'm out of movement points. Crap!"
