Take Your Pick
One method for doubling on birds calls for the first shooter to count "1, 2, 3, BANG," with both guys shooting on "BANG". To be honest, I've never pulled this one off (or seen it done successfully, in person). I've heard a bunch of stories of it working, but there are even more stories about it going awry. Usually, one guy shoots too quickly. Then his buddy is caught off guard and either pulls his shot, or never gets to shoot. Sometimes the hunters can get so caught up in getting two good shots that neither one gets a shot opportunity at all.