Appointments+ Avoid bookings in 2 calendars?..

I have Appointments+ set up for a customer, it works beautifully.
However the customer now wants it to avoid bookings from 2 different calendars.
Both of these calendars are set-up in the same Google Calendar. And one of them is the one already used by Appointments+.
Any thoughts on how I could get it to avoid pre booked slots from the other calendar as well?
Thanks
Dan