For a male to wear a strap-on on his ass, enabling him to have sex with two girls simultaneously in a thrusting motion, swinging his hips, penetrating the
two females vaginally or anally.
Can be done either laying on
one's side with two girls straddling from the front and behind, with the male penetrating both females in a back and forth motion.
Or
Two females mirroring each other in a doggie position. The male will
face towards
one female and thrust his groin back and forth so he enters both
one after the other.