How to to stop double booking with two calendars.

I have set up 2 x Appointment Types and 2 x Calendars. 

But, I keep getting double bookings. 

How do I stop double bookings when using two separate calendars inside Acuity?

Any help is appreciated. Thank you. 

I'm having this same problem. Are you having them sync to the same external calendar? 

I ended up setting a "resource" of 1 room for all the appointment types that used those calendars, and that solved my problem.