You can only subscribe to one subscription per app for an Apple ID. Apps you purchase from the App Store can be installed across all your iOS devices as long as the iTunes account associated with those devices is the same. 

Please make sure that you have the same Apple ID in both devices. You can also check under your Apple ID > subscription that you are subscribed. In case they don't appear, i recommend you contact Apple support team as payments are sole managed by Apple. We don't have access to that.