Once a customer has picked a service, the time step offers your other services under Add another service to the same visit. Adding one changes the times on offer to fit the whole booking, for the people who do every part, and the price and length add up. The diary card, their confirmation and the emails all read "Haircut + Beard trim". Price options belong to a single service, so a combined booking uses standard prices.
Under the times, Join the waiting list lets a customer put their name down for a day, for a person or anyone. You see the list on that day in the diary. When a booking on that day is cancelled, the first suitable person on the list gets a message (and the email nudge) with a link that holds the freed time for two hours; one press books it, with any deposit or form asked as usual. Untaken, the time passes to the next person, and so on until nobody is left.