You could script an enemy vs enemy attack, but the built in AI system which automated targeting, path finding, etc only allows enemy vs player right now. We have the ally and bad vs bad in there, we just need to expose it via LUA and through more entity controls, which will happen when this request floats naturally to the top of every ones wish list.
PC SPECS: Windows 7 Ultimate 64-bit, Intel Core i7 920 (PASSMARK:5008), NVIDIA Geforce 9600 GT GPU (PASSMARK:752) , 6GB RAM