Can I Use 7 Minute Workout On iPhone And iPad Without Paying Twice?

Yes. 7 Minute Workout Pro works on your iPhone and iPad at no extra charge, as long as both use the same Apple Account.

Follow this guide to restore your purchases.

The same goes for the Apple Watch and Apple TV apps: Pro on your Apple Account works on all of them. Your workout history and custom workouts sync between your devices through iCloud.