I've been doing a lot of Duo's and Trio's in PP2 with Morais, Sungsee and xXCrashXx, so I figured I'd share some of what we've learned in the 3 mobs. If you're in a full party, this shouldn't even be an issue (although I'd suggest finding a duo/trio/quad and get almost twice as much xp).
There are 2 groups of aggro in the 3 mob rooms. Group A has mob A1 and A2, where Group B has one mob in it. The groups should be apparent, as the names are the similar on the linked mobs. There are a number of potential situations to find yourself in after killing a mob, I'm going to give my perspective on all that i can think of.
Situation 1 - You've just killed mob A1, and both A2 and B were alive for some of the damage delivered A1. The "correct" response is to attack mob B. The reason is that since A2 was alive/spawned for part of the damage delivered to A1, the tanking character will have aggro built against both mobs in group A, and still needs to control the aggro for group B.
Situation 2 - Again, you've killed mob A1 and B is alive, but A2 has just respawned or for some reason is "un-aggroed". This is a tougher call, my suggestion would be to chip/pull the one closest to your squishy character and leave the other one alone (watching it to make sure it stays neutral until you can grab it) or grab both of them with pulls and ONLY chip one. If you pull both with chips, you are likely to confuse your party members and end up with a half dead healer, or a fully dead pixie. .
Situation 3 - You've killed B, and both or one of A1 and A2 are alive. This is the second easiest. The only thing to worry about here is not to miss your dual summon if you're a pet class
Situation 4 - There's only one mob to attack. This is the easiest, and you're killspeed is about perfect for the room. GO YOU!!!
(just for your squishy party member's sake, watch out that you take aggro with a chip or a pull in case another mob spawns. I'd rather have to throw one extra small heal on a tank than 2-3 on myself.)