Sharing same userbase with different forums

[Caseyjr86]

I have this website, a starcraft 2 dedicated site. I'm thinking about starting another product of blizzard site, Diablo 3. So I have a few questions.

1) How hard is it to use different domains, but share the same userbase/posts and forums but have a different default skin for each website?

2) Would this require me to purchase another license?

[Dismounted]

1. Search for the "Different Domain Different Style" modification.
2. If you don't hide forums based on domain - no.