Well, as it turns out, they both kinda got stuck on each other when I went back to proc the fight again, and I was wearing a full set of troll leather armor so I think my ability to be Sneaky let me hide near a rock at the bottom of the hill and just launch fire arrows up at them. They were just staring at each other as they were burning, then when one fell, the other one simply stayed in place until he also fell over too.

Maybe if we fought the boss right, we could have gotten some excitement, but given the dangers of the double-summon the first time, I'm kinda glad it ended anticlimactically.

Moral of this story: ALWAYS make sure you and your friends know who's summoning the Forsaken Beast, and NEVER forget to fight dirty!