If you have the same Android or Apple account on both devices then you only have to pay once. If you have separate accounts (either two Apple/Android accounts or a Google and an Apple account) then you have to buy the App twice - this is controlled by Google. Amazon, Samsung and Apple, as they handle all App accounts and payments (this is the same for all paid Apps).
Either way, you can (should) use the same iFootpath log in details for both Apps and the website so that your comments, gallery uploads etc are all associated with one account.
However, Apps on different devices do not talk to each other. So if you download a walk on to one App on one device it will not automatically be on the other App - you will have to download the walk again. To make this process a little simpler you can tag walks as Favourites or Completed.