With one purchase you can use iReal Pro, and any backing-track style packs you bought, on all your iPhones and iPads. Sign in with the same Apple Account and download it from the App Store on each device.

 

To bring back purchased style packs, open Settings > Store and tap Restore Purchases, signed in with the Apple Account you used for the original purchase.

 

For more, see Buy iReal Pro and use it on your devices on the iReal Pro website.