Same concern here - if my wife and I can’t both get the cosmetics and play all content on the same console at the same time, we’ll skip the DLC altogether.
As a suggestion, if Player A pre-purchases the DLC, Player B should be able to access everything that A can as long as A is playing with B. You could block B from using the cosmetics and playing the DLC when not playing couch co-op with A, in other words. If B wants those things independently of A, only then would it make sense to have B purchase everything separately.