How to synchronize users between two buddypress multisite installations?

I’m trying to connect two separate multisite installs — I’d like registered users to be able to seamlessly pass from one to the other. The catch is that both installations are prosite installations BUT I don’t want prosite capabilities to be synchronized — just the buddypress capabilities (such that a user could have a premium blog on one install but not on the other.

I know, I know… Just thought I’d ask.