If they both transform is there an argument for being a TOTAL BEAST and bringing them both down evenly? So bring them both down really low then kill one. That way when the other transforms you only have to deal with a very small percentage of his HP? That's what we used to do in various World of Warcraft raid bosses who had similar mechanics to this. I mean it would be harder as you'd be spending longer with both bosses alive, but that dual phase didn't look too bad apart from the odd time when rng meant they ended up right next to each other. If you can keep them split up it might be doable.
I wonder whether the other one regens HP though when his buddy dies to stop you doing just that tactic. Again, WoW bosses sometimes did that too!