2 On 1 on LetMeJizz
Two performers working one in the middle, the frame catching all three points of contact at once. Most often two guys and a woman, one in her mouth while the other takes her from behind before they trade. The lens pulls wide to keep both cocks and her in shot, then cuts tight when the focus narrows. The math is the whole appeal, one person kept busy from two directions with no idle moment.
141 2 on 1 videos here, all HD and all free. New ones go up daily, so check back if you don't find your scene first time around.