Installing app on multiple iPhone devices and charts cannot be synced

If you have the same Apple ID on each device, then you should not have to pay again. 

Go to the App Store on the device you want to put the TimePassages Pro App on, then go to purchase the TimePassages Pro App. With the same Apple ID, Apple will remember your original purchase and you should not be charged again.

Note: The charts that you created on your iPhone cannot be transferred to the iPad. You will have to enter all of the charts on your iPad manually. You can backup your iPhone and restore it from an iCloud backup onto a new iPhone device if you have the same Apple ID, but you cannot sync onto an iPad.