Can I use the PianoTrax app on more than one mobile device?

Yes, you can use the PianoTrax app on multiple devices (just not at the same time) as long as they are both/all Apple or both/all Android.

Use the same email address and password for the PianoTrax website and the PianoTrax app.

You can only be logged into one mobile device at a time. As soon as you log in to a new mobile device it will kick you out of the previous one.