No need to multiple buy. I explained how to do it some posts ago.
1.- On Device with Apple ID "A": Download App, login with your Nintendo ID ("A"), & buy the 9,99 full unlock.
2.- If the person who you want to share your purchase with doesn't have a Nintendo ID, create one on accounts.nintendo.com.
3.- On Device with Apple ID "B", sign out from the App Store, and sign in with Apple ID "A". Download the game. Open it. SKIP NINTENDO ID CONFIG, complete the tutorial and press to buy the 9,99 full unlock. Say yes to purchase and introduce password. It will say you've already downloaded it and that it will be free. Then link the game with the Nintendo ID "B" account or the one you've created. This will tell your "B" account that you have unlocked the full game. Wait one minute so it syncs.
4.- Close the game, delete it. Sign out from the App Store. Sign in with you Apple ID "B" account. Download the free game. Open it. DURING setup, link it with the Nintendo ID "B" account. To link it, if it only offers to create new accounts, clic on the top left where it says Nintendo account which will bring you to a login page. After login in, press the blue OK at the top and an error will appear. Ignore it. Try again the Nintendo ID linking and know the page that opens will say that you're already logged in and there will be a big button to use the account with the game.
5.- And you're all set. You'll se that you have the full game unlocked.