-noun; a reference to the afore-mentioned aspect of copulating with one sibling and then the other at a later date, spoken of as though it were a status, achievement, or standard to be gained
(noun) "One of my lifetime goals is to achieve a Double Down with identical twin blondes, giving one a Tony Danza and the other a Bareback Surprise. All together, I will have achieved the trifecta of sex maneuvers, thus sealing my name among the gods forever."
"Dude... that's really gay..."
"Nah bro, these dancing pecs get me so much pussy at the bar."