Just thinking about this for a minute or so, I don't think there's a way of allowing /only/ one door to remain open-able ('unlocked') that solves the problem. Either both must lock/unlock in the same action or there must only be one door to the restroom (that is, duplicate the resource).
It's a different problem.
If the two people are accessing the bathroom from the same side, locks are not a problem. Three or more people sharing the bathroom is a different problem.