While you can download the app to as many devices as you would like, JellyTelly only supports one device playing at a time, with the exception of a tv and another device. So if you wanted to play from your tablet at the same time as the tv, that shouldn't be a problem at all. However, you can't play on your tablet and say a laptop or phone at the same time.
To simultaneously play on two tablets, phones, or laptop devices, you'll need to create a unique subscription for each one.
We hope this helps! If you have any other questions about how this work, send an email to firstname.lastname@example.org, and we'll be happy to help!