Do not worry if you have multiple iPhone/iPad devices. You do NOT need to pay for twice. It's also free for you to use on other iPhone/iPad after you purchase pro version once.
Just click the purchase button again on your another iPhone/iPad, remember to use the same account as you purchase, at first it will ask you confirming to buy, go ahead(no worry, you won't be charged again as long as you use the same account)it will say you have purchased, do you want to get it free? select yes.