Dual membership means that the tribunal becomes a separate flag for your character. In your score, you have religion, kingdom, cabal, race, gender, profession, et cetera. You would have another spot that says tribunal. One could join the Hammer and the Peacekeeper at the same time.
Code:
Hammer Peacekeeper
Crucible Hand
Fist Talon
Druid Guardian
Harlequin
Some combinations do not make sense, but could in theory be accomplished. Midnight is a unique circumstance. I could see Harlequin being a in a tribunal, but I would imagine only for sabotage, personal gain, etc. Midnight would likely take up both slots. So in theory you could have a Hammer Hand. I don't see it, but who knows. That's probably the one which disturbs me the most, but I believe we'll tend to see players policing themselves. They often do a better job of that than we can. Simple peer pressure does wonders. Tribunal membership would run alongside your kingdom of residence and cabal membership allows you to provide that cabals influence in the kingdom which you reside. Its funny. Anyone read help Shattered Kingdoms lately?