2 Multisites Sharing Same Login Credentials

Hi,

I'm intending to create 2 multisites - say Multisite A and Multisite B.

I'd like to have these configs :

- Signing up via Multisite A
- Log in of Multisite B will be that of the same credentials as A

There will not be any signup or user administration via B. Meaning, B users will log in to B via same username/password as A.

How can I let B share the same database of A? Is this possible?

Please advise.

Best Regards,
William