2 players riding a boat into the world border will make the boat split into 2 boats, giving 1 oak boat and 1 of whatever color. My theory for this is when it tries to teleport the players away from the border, it thinks the players are riding individual boats, not sharing one. So it moves the boat away with the players, spawning another boat since it thinks the player is riding one but it doesn't know it can put them inside the same one. Or I could have no idea what i'm talking about. Probably that second thing.