During the Brevlik telescope quest, I killed one of the Flaming Fist Enforcers that arrives in the House of Gond (AR0130) after filching the telescope in plain sight. Everyone went hostile other than bystanders, including the nearby Be'land (BELAND.CRE)- so far, ok. But then I noticed a Flaming Fist Mercenary swinging at Be'land, and once Nexlit the Blue Elf went Inviso, it was a fight between the Mercenery and Be'land, mostly instigated by the Mercenery, mind you. The Enforcers didn't take sides. For sure Be'land goes hostile at the same time as the others, but he doesn't seem to be allied to them.
I just reproduced it in BG1 as well by simply telling off the arresting officer after being seen taking the telescope- no killing necessary.
Also when the mass hostility happened, Brathlen (BRATHL.CRE) stayed a blue circle and started fleeing like the rest of the bystanders. This is not the same as BG1, however, where Brathlen also went hostile and started casting Clerical spells.
There seems to be a chance for Enforcers to attack each other as well, usually when getting in each others' way trying to go after the party member.
Be'land, Brathlen, and the Mercenaries should all be on the same "faction" as the Enforcers and not attack each other.