You can use the same email for different storefronts using either the same or different passwords. However, there are a few rules.
You cannot have two End Users in the same Storefront, or two Administrators or Suppliers with the same email.
You could have the same email in 2+ Storefronts.
You could have an Admin user and an End User with the same email address. Where they go at login time, will depend on what URL they use and what password.
Admins
Admins can log in from anywhere and always be directed to the admin site. Unless
Admin is trying to log in as an admin using a storefront login page they already have their email registered in. Admins should avoid any issue by always logging into their section, which always looks like this admin-systemname.printjob.com (unless using their domains)
End Users
Users can access their site from any login if their email only exists once in the system.
If a user has the same email in two or more storefronts, they must log in directly with those storefronts.