That’s not a problem at all. You can use Portal on both your iPhone and iPad without paying for Premium twice.
As long as you’re logged on to both devices with the same Apple account you’ll be able to unlock your purchase on your other device as well.
You can access Portal on your other device by following these instructions:
- If you haven’t already done so, download the Portal app from the App Store onto your other device.
- Tap the “Restore Purchase” link below the buy button and you’ll be able to enjoy Portal on this device too.