They have never gone into detail They aren't that far along that it has been hashed out yet.
We do know that most racial inactives will stack with buffs...but wont stack with themselves.
From the little bit that has been mentioned by Brad and others I think you can take early EQ as a very general basis for comparison. That being that stronger buffs of the same type will generally over write a weaker buff of that type. Basically common sense as most people expect them to work.
