Two Themes, One Blog

I’m currently setting up a simplepress forum for my blog. I really like the idea of users who sign up for my forum also signing up for my main blog. I think it will definitely help with community building.

I know I can just install the forum into a subdirectory/domain, but then the signed up users would only be signed up for the forum or blog and not both. I definitely need to have a separate theme for my forum though…

Is it possible to install two different themes on the same wordpress install? Or somehow combine the users from two different installs automatically?

Jim