If you know there is a witch that can eliminate emotional bonds within range and you currently do not have a particularly good reason to trust them then you kill them.

Both Addy and Chelsea have extremely good reasons not to kill witches; Addy can copy their power, and Chelsea can convince them to use their power for her own purposes. I suspect this would be enough to lock them in a stalemate. The battle between rebels and Volturi would look interesting - in the midst of wolves and vampires duking it out, you have two vampires who aren't on anyone's side, and who don't care to be on anyone's side.