You may have as many SIMs as you wish for different devices, but each must be registered on its own account.
To register an additional SIM, here and enter the 6 digit activation code from the new SIM. Click "Activate your SIM", then register a new account (do not log into your existing account) The member name must be unique and will be generated automatically. Everything else (Name, address, email, password, payment card) may be the same if you wish.
Order additional SIM cards