[KuraFire]

Interesting.

Not exactly what I'm thinking of for using for myself, but this might actually come in handy at one point or another, nonetheless.

Currently, my plan is to make a network of all my sites. Two of those sites will be run on a different domain (probably stacked on the same server on the account of my first domain, though) with their own databases, but since my forum (the first site, or at least, part of it) is a support-forum for both of them, I want to design user registration for the two other sites in such a way that, with a single checkbox, they can choose to immediately sign up for the forum At The Same Time, with that very one signup session. So they'd be added as a new user to TWO databases - for this, I will have to do something similar to what you showed up there, yet in case I end up with my three domains on three different servers, then I will probably use the above for some extra connectivity.