I think many things have to be taken into consideration if you are even contemplating double booking.
Biggest one for me is ~ do you have more than one parking area if both are there at same time, I wouldnt want either of my clients seeing each other, so two parking areas is really required.
I would never do it for bookings of longer than 30 mins, many guys are willing to wait for a short time if you tell them you are running late
and will be with them as soon as possible and thank them for their patience. I would also offer a small discount for their patience, after all if you are getting the two bookings in then you can afford a little 10GBP discount.
Sometimes you have to think on your feet so if you are no good at that then double booking is no use for you.
So long as you sound upbeat and bouncy guys are usually accommodating. Its also handy if you have a garage or something similar where they can get a coffee and the discount will cover the cost of the hot drink.