If you use the same Google Account, you only need to sign up for the Premium Plan on one of your devices.
On the other device, open the settings screen and tap "Premium Service" → "Restore Subscription". The Premium Plan is then applied there as well.
If it isn't applied, make sure the Google Play Store app on that device is signed in with the same Google Account you used for the purchase, restart the device, and try again. If it still doesn't work, please contact us using the "Contact Us" button on the help screen. Thank you.