Unfortunately, the purchase of Forest made in either iOS or Android is not transferable. Apple (App Store) and Google (Google Play Store) sell the apps in their own market and handle all the selling process. They are completely different platforms and thus the purchases cannot be transferred from one to the other.
As app developers, we grant distribution rights of our apps to the platforms but we are not in charge of the app-selling process. We are really sorry that there is nothing more we can do regarding this fact and we cannot exchange or give you either the iOS/Android version of the apps. (We would like to if we could.) For this reason, you would need to purchase the iOS and the Android version separately. But once the app has been purchased in the App store or in the Google Play Store, you can download it on multiple devices for that certain platform.
Last but not least, once you have registered your account in Forest, you can definitely use the same account on different devices. 

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