I am adding an upvote to this idea.
In our situation, we do interviews for employment candidates with a team of two. We'd like to have an option in the Staff Assignments for a Service for a checkbox that says something like: "Require each staff member to be available to book an appointment" or something to that effect. When checked, potential candidates would only see time slots where both staff assignees are available. If either one has a "busy" on their calendar, that time slot would not be available for a booking.
I should mention that we turned off the ability in the service for "customers" to select a staff member since when it is turned on, they can only select one staff member. When an admin sets up a booking, they can select both staff members, with feedback about each staff's availability.