You certainly can!
Simply Piano and Simply Guitar are available on mobile/tablet devices (we do not have a desktop version).
Once you install the app on an additional device, make sure to press the ALREADY A MEMBER button on the opening screen of the app.
Then, enter the email you used previously, in order to continue.
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Have more questions? You can talk to us through the app. Tap on Menu > Settings > Have a Question? :)