I don't think two showers is a problem; the problem is that he's expecting them outwith his booked time. Just make sure he's out when he should be (so if there's five minutes left, ask him if he'll be wanting a second shower and if he says yes get up and stick a robe on/direct him to the bathroom).

If he whines about it tell him that your next punter will be here in fifteen minutes and ask him why he only booked thirty minutes if he needs longer? After all, it's no great loss if he doesn't come back

I've had a few like this, they rely on the fact that you'll be too polite to tell them times up.
You can suggest a longer booking but I doubt he'll agree to that as basically he is a cheapskate who just wants free time.
It really used to wind me up too and in the end I stopped taking their bookings, there will always be other better behaved clients to take their place.
You could give what Amy suggested a go if you want to give him one more chance, be very strict and get up and robed whether he has finished or not. If he doesn't like it he may not bother you again so win win, those types are really not worth ruining your mood and your day. 

I have a reg that books half hour and has two showers. Does kinda piss me off but he doesn’t attempt to overstay. If he tried, he wouldn’t succeed. At least you know his game now and you can place those boundaries by wrapping things up in good time.