Many salons have booking systems that allow them to have a drying time in the middle of an appointment and a few of the booking systems allow you to program this in when using their appointment systems.
Eg. You book you self in for head of highlights and a colour. In total this may be 2 hours but your actual time with the service provider is 1:30 and there could have been another appointment slot available.
So i would program the appointment to make a 1 hour appointment, then 30 minutes after the end of that another half hour one.
Linking services with rules that determine how close they can be would also be an option however when you have two appointments the customer gets 2 email reminders and this winds them up.
Is this something that could be built into this plugin, I think you would really clear up with the salon market with it.