appointments at specifi time

I am struggling with how to configure something.

My client has 2 conference rooms
My client has 1 mediator

The conference rooms can be booked with or without the mediator.
The conference rooms are available EITHER 10AM - 1PM, or 1 PM to 5 PM.

I cannot allow 9AM-12, or any other time slot.

I know I can set service duration to 240 minutes, but how do I have it start at either 10AM or 1PM?

J