A purchased version of smartChord can be easily reinstalled or used on multiple devices. You don’t have to pay twice. Just install it on your devices. It is important to use the same Google Play account with which you have bought smartChord. If it was an in-app payment, that’s enough. If it was a separate s.mart app, just install it on your device. You can find your bought apps in the order history: https://play.google.com/store/account/orderhistory
You can check if everything is installed correctly:
‘Info’ -> ‘Your products’.
Your purchased products are listed there.