Users are shared across tenants, so if you have one person logged in and they visit a different Fusionauth application url, they'll be automatically logged in again (without the need to auth again). I'm not sure that's what you want.
If you register a user to app A and then to app B, they'll be registered in both apps.
If you register a user to app A and then to app A again, I think you'd get an error message, but would have to try that out to be sure.
If you want to try this yourself, you could try richb201+1@gmail and richb201+2@gmail (anything after the + is treated as the same email address by gmail, but different email addresses by fusionauth).