As a 3 boxer I turn in fully on main alt if applicable, then alt+tab, then alt+tab twice so that the alts are switching when I alt+tab; I hit npc, alt+tab, hit npc on 2nd, continue, alt+tab, continue, accept/complete quest, alt+tab, accept/complete quest. Yeah it's a pain in the ass but it's the only way to do it when turning in quests; you CAN take the quest on one Character if it's shareable and then share it and use a macro to accept on all alts at once, but I feel more comfortable tabbing personally and knowing all the chars got it lol