Thing is, dretches just walk into things to damage them. If one dretch hits another, then the other guy hits him too :roll:
The game can't possibly tell between one guy hitting the other in the head or the other way around, so they'd both get the headshot.
Only thing I can think of is how apparently you need to be looking at the enemy to actually damage it, but if both players are spazzing on the walls... argh. It makes me sick thinking about it.