ICE has a post on this somewhere in Strategy, but I can't find it. In General, If 2 ninjas give the same buff for the same number of rounds, then they won't both apply. So, if you have a ninja that gives 30% NIN boost for 3 rounds for Akatsuki, and another that gives 30% NIN boost for 3 rounds to all Lightning ninja, only one would be applied since they both last for the same # of rounds. But, if one is for 3 rounds and one is for 5 rounds, then they both will be applied.

yeah, that is the thread I was trying to find.