Reparation + AoE taunt (Righteous Imperative) every 60 secs + they have Mein of Leadership which grants +450/90/100% to threat/endurance/armor, respectively. Once they have threat, you just about can't pull 'em off. I do tend to rip AoE aggro if I blow too many AoEs on the alts but only if I do it too soon or if Righteous Imperative is down.

I suspect Reaver DoTs spreading around might be slightly better at sustained AoE aggro but I don't have enough experience with mine to really say for sure.