It is not possible to be logged in with the same user on two different devices. If you log in with the same user on another computer, mobile phone or tablet, the other device will automatically log out.
There is one exception. When scanning tickets, you want to let in as many people as possible, as quickly as possible. To support this, it's possible to have multiple active logins with the same user on mobile devices used for scanning.
This applies when you're scanning tickets using the Stager mobile app.