You can have the unlocked version of Waite’s Guide on as many iPads as you wish as long as they all use the same account. Apps are tied to accounts not devices.
Once you have unlocked the app on one device on the 2nd device go to the app store and find Waite’s Guide or look for it under Purchases in the app store. If you are signed in to the same account you will see a small cloud where the price usually is. This indicates that the app is stored in the iCloud and can be downloaded for no charge. When you have finished downloading your version should be unlocked. If it is not go to the Unlock button on the Home page and tap it. When the message box appears tap the Restore button. It will ask for your username and password and when you press OK the app will close the in app purchase screen and all the data will be available.