Sharing Same User Databases/Log In Details between 2 Multisites

Hi,

Is it possible for 2 multisites to share the same user db/login?

For example, multisite A is the main one that handles the signup/registrations.

Users can then log in to multisite B using the same credentials. Then they’ll create their own site populating blog.dir. Multisite B will not handle registration.

The reason why I want this set up is because the 2 multisites handle very different niches but I’d like to offer them altogether as a package.

Thank you in advance for the reply.