Create new save, and new character in sandbox. Add player 2 on player 1. Quit game, reload, player 1 is now player 2's character, player 1's character data is now missing.
If you create a character, then exit to menu it will correctly save your character data and you can load back in and add player 2 without deleting the first character. Not sure how that works but it's consistent.
Build 41.30