My idea is: When you find a collectible on a friend's island, you get 50% of the collectible.
For example: If you find a scarecrow, and it gives 2 scarecrows, your friend gets 2 and you get 1.
More examples (numbers):
Friend - You
1 1
2 1
3 2
4 2
5 3
6 3
...
What do you think about this idea?
