The siamese twins are not the weirdest part of this screenshot. The other guy in the picture is functioning as my shadowguard. I had hit two of the 3 players there with poison cloud and presumably they had enough dot to die, but their hp's didn't go to zero. Instead, I simply couldn't use any skill on them. So, I placed my shadowguard next to one of them, and both mob and guard disappeared for a second, and then the guard reappeared (in terms of function) but the appearance was that of the mob. As for the two mobs sharing one hex, one is dead and the other alive. When I hit that hex, the fight ended.
That siamese twin effect was new, but the zombies have occurred in nearly every fight my lvl 28 rogue has had with that group. It doesn't seem to happen, however, if poison cloud is not the FIRST skill I use.