[Organizers] Bookings etiquette

(tap, tap, tap -- is this thing on?) A bookings etiquette question: For a special contra event (weekend, daylong event), is it better to ask performer's availabilities in serial or in parallel? In other words, should one focus on one bookee at a time, only moving on after a non answer, or a reasonable amount of time? Or is it better to ask multiple talents at once, and then select from those that responded? Curious, (and also curious if anyone's still on this list) -Chris Page San Diego