My opinion is that the lock on the outer door was put there with the intention that it would be used. Why there are two stalls is intriguing. I've seen it that way before too, but I never, ever saw two different genders use it at the same time. To my knowledge, the outer door
always got locked.
Personally, I would lock the outer door. The previous posters whe mentioned the risk of an attack aren't being overly paranoid. It happened in the bathroom I use daily in our building, to a woman who worked in another office upstairs. Turns out it was a guy who was on work release, already in trouble for attacking a woman, and he wasn't after her purse (we know this because she threw her purse to the door hoping he'd grab it and run out, but he didn't. He had her trapped in the corner of the bathroom and he kept coming at her.) She somehow finally managed to escape out into the hallway and get someone to call the police, and the guy was arrested not far away shortly after that.
After this incident, everyone was on high alert, and we were told to notify the police immediately (as we have no security in our building.

) if we ever see a male entering the women's bathroom.
If it were a unisex bathroom with a lock on the door, who knows what might happen to some poor girl in a similar situation! I think it certainly adds an element of "opportunity" that really doesn't need to be added.