SMF/vB Shared Database
 

I've never really seen it asked (or even done) before. Can you get two forum softwares to share the same database? Possible, even if difficult?

No problem at all, as long as you use prefixes to ensure there are no tables shared between the 2.

I imagine, then, that there's no way to get them to share threads and posts, which is what I was going for?

Lol, no that would be impossible (or loads and loads of work).

Sounds like a job for thousands of cheap data-entry users.

Is there a bear minimum of information that the two could share besides post and threads? Say, just login or account information?

I've build an application that does this sort of thing, but haven't made it work with SMF before.

I'd quote this with a strong "far more trouble than its worth". Why do you want both forums running simultaneously?

I wanted a styles demo forum that didn't require a user to register on each forum to demo a style for a respective software.

So just allow guests to change styles.

That still limits them to guest view. I want them to be able to physically interact with the forum as a registered user. I don't plan on opening up posting privileges to guests, so I would like a basic solution to the login (and ideally posting) that would allow them to move around freely without multiple registrations, particularly if I decide to expand designing skins for other forums softwares.