So is the problem that you keep teleporting to your friends island instead of yours? I haven't played with any friends yet (forever alone ) so I'm not too sure about this but tell your friend to kick you off that island. Maybe you can only have 1 island, either your own or half shared with your friend. I'm not too sure about this though.
exactly Dude, you don't get it. i am on HIS island. and he made me a leader. but when i type /is i get tped back to my old island
It is a cross-server bug. Your first island that you started with, if you don't delete it, if you joined another server and made another island there with a friend, when you do ./is it will teleport you to your first island.