Would this mean that if one suffered two different necromancer attacks at once, the brought zombies could attack the other necromancer's zombies?
Probably, but I have not seem two necromancer's sieges at the same time, so I can't comfirm but that is possible (also, it's most likely they will be passive to eachother, since the necromancers would need to be at war with eachother to that happen...) since they may attack the living soldiers of one necromancer, and that trigger the aggression of the zombies, then everyone on a full fledged fistfight.
This doesn't seem true anymore. In my old necromancer fort the undead killed everything except other undead, but recently this has not held. E.g there is a thread (and Ive had it happen too) with numerous examples of zombies showing up with elves or humans, living and not necromancers, but allied. It seems Toady rewrote some aspect of undead loyalties and now they are no longer hostile to the living in their own faction.
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This makes sense, but has anyone tried to see what happens if a member of your fort does the raising? It might remain friendly...
Zombies raised on site by necromancers still follow the old pattern, and will attack the necromancer's allies, resulting in an epic loyalty cascade. This is probably a bug.
Sadly, this applies to undead raised by your entity ruler too, his zombies aren't part of their civ/group, they are "normal" zombies and will attack your citizens.