I'll admit that me and the healer (we're friends so we do dungeons together) did that thing for the first time. We usually just kill them. So I was a bit inexperienced in metal gearing those two dullahans. The tank just didn't handle the situation right. He didn't target and the other dps just was overwhelmed.
Here's the kicker:
After we wiped the tank is all "Whoever pulls aggro again has to walk back" Are you kidding me? How fast and desperate do you want this dungeon to be over with? The tank plays a vital role in a group. He needs to have a contingency plan should plan A fails.