none of the pilots will need to leave the cockpit area unless, they are on a wide body, long-haul flight where there is a relief pilot. Even then, the relieved pilot does not leave until such time the relief pilot is in his seat, which is normal procedure anyways. (I would think!)
Trouble is you've got types such as the 777 doing 8-9 hour 2 pilot sectors...and on some variants there's no loo forward of 2 left/right so plumbing an extra toilet in up front would be non trivial.