Bookings Service assigned to multiple staff

How do I set up bookings so that if I have two staff members, when someone chooses a service, that it goes to both staff members?  However, I also want it setup so that, if only one person is available at that time, then it can still be assigned to that one person.  So basically, if both staff members are available, then the service goes to both of their calendars.  But if only one staff member is available, then it will go to that staff member.