The PianoTrax app and website work hand in hand. You can be logged into one computer and one mobile device at the same time. They both use the same email address and password. If you are having trouble logging in, you may have to reset your password on the website.
You can have the PianoTrax app on multiple devices, but you can only be logged into one at a time. You can share a subscription on more than one device, but an Apple subscription will not work on an Android device and vice versa.