With the Multiple Site Manager, membership is installation-wide. You can set permissions, and even the member-list, per site and per group; however the member accounts will work on all Sites. If you need a fully separate user-base, then two installations (and therefore licenses) would be more appropriate.
The MSM is intended for related internet properties, that share a membership base, and share the same owner. It’s also important to bear in mind that there is no way to split a site out of the MSM, so if you did eventually want to separate these sites, starting separate is best.
Make sense?