Train.

You slip from beneath Jenna, and she's so insensate right now that she barely notices. You climb around to behind the thrusting Zoe.
"What are you doing, bad boy?" Zoe takes a moment to invade your mouth with her tongue, redoubling her efforts to fuck Jenna into the bed, mercilessly. Her backside looks so lewd with Zoe's fake cock snugly in her snatch and the jeweled end of a buttplug wedged in her backdoor.
"Giving you a boost." You line your cock up with her snatch, and she leans forward, to facilitate your entrance.
"I don't think it'll work very...." but you aren't trying to fuck Zoe's pussy. "ooooooooooooooOOOOOOOOOOOOOOOOOHHHHHHHHHHH SHITTTTTTTT......!!!" Zoe shrieks as you spit on your cock and push your way into her tight asshole.
"Annhhhhhhhhhhhhhhhhhhhhh!!!!" Her whine is half pain half ectsasy. Once you've loosened her up, you really start going to town, and your thrusts transmit straight through Zoe's ass, into the strapon and on into Jenna.
"HOLY SHIT?!?!" Jenna starts babbling again... "That's amazingomigodomigodfuckfuck...." Zoe...poor Zoe...you peek at her face, but her bell is rung. Her eyes are rolled up and she's drooling. You grip her hair and bite her earlobe.
"Wake up, little Zoe. You're getting fucked in the ass by Big Game James." ugh.... did you really just say that?
"Unthhhhhhhhhhh....." her eyes flutter.... "so.....thicccc. You're thicker than Big Tim...."
That gives you a moment's pause. Big Tim Davidson is the owner of the Black Diamond club, and the Black Velvet strip club where Zoe used to dance. A worry for another day.
Jenna cums, squirting all over the bedsheet and screaming with ecstasy. Zoe pulls out and falls sideways with you, and you pull out of her hungry ass. Jenna lays on her side, convulsing.
After taking a moment to wipe down your dick with a sanitizing wipe, you re-lube. You're taking Zoe's pussy.
As you pound Zoe missionary style, with her absurd fake cock still flopping between you, Jenna comes behind you on shaky knees. She pushes on your ass and kisses your neck, then your mouth.
"Fuck her, baby...Fuck your sister." The words, coming from Jenna's mouth, are exactly what you've always wanted but never knew.
You can feel it building. Jenna lays flat next to Zoe, and you slide fingers into her cunt.
"Are you...." Jenna start moaning.
"Soon..."