Sort of, only not exactly like you describe. Users don't register for a specific blog: they register with the installation. You as admin decide what group you want to put them in and what blog(s) you want them to be allowed to post into. You define blog membership on your blogs tab (choose a blog and pick the permissions subtab).

Ah, thank You EdB, for Your answer.
So I know that I better look out for another Software.
Or maybe I choose two Installations of B2evolution....*thinks*

Thanks so far:
Bastian

You can, if you're into it, have multiple b2evolution databases in the same database by changing the table name prefix in conf/_admin or conf/_advanced. I forget exactly which one, but there is a variable for the prefix name that defaults to "evo_". There is also a variable for the cookie name which you might want to change as well. I believe it defaults to "b2evo", so you might consider b2evo1 and b2evo2.