clusterbooking in the same time slots with online booking

The option to clusterbook pt while they book online? I want to use it but only if i can get double bookings in 1 timeslot first. So. if i have a pt at 16.40, 16.50 (double slot already filled), 17.00. I would want them to book at 16.40 and 17.00. Not at 16.30 and 17.10 first.