[gruseom]

"Distinct divisions of power" strikes me as an idea that might be more trouble than it's worth.

Either there's something about your partner that's bothering you -- in which case you should address it explicitly and not put it off -- or you might be bringing worries about power, conflict, etc., into this situation that actually originate elsewhere, in which case you should get aware of that and make sure it doesn't poison your relationship. Either way, the technical legal level is probably the wrong way to approach this. If you can't trust each other, no document is going to save your business.