Pincer movement

Dr. Black leans over you, squirting a liberal amount of lube over each of your cocks. She swings her leg over your waist, her knees now either side of your lower torso as she mounts the chair.

She grasps your lower cock and smiles at you as she guides it while she slowly lowers herself down over you. As the tip presses against her tight rear hole she bites her lip, you feel the pressure increase as she tries to **** herself around your lubricated cock. A horny frustrated little moan escapes her lips as she fails to split herself open for you. You reach out and place a hand on her hip, your thumb makes soft circles on her lower body.
"Relax doc, breathe."
Dr. Black nods, closing her eyes as she rests her weight on her legs and the hand which isn't guiding your shaft. She breathes in deeply, making her breasts raise in that enticing way. She breathes out and lowers herself down eyes still closed. The pressure on the head of your lower cock increases but you can feel the difference this time. As she presses down your head opens her slowly, and suddenly there's enough pressure that you slip into her. She makes an adorable little squeak, then smiles at you in thanks. She moans softly as she slides down you, past the difficult point now as she fills her ass with your thick cock. Her wet pussy rubs against your other cock as it slides down, leaving you outside, but coating it with her juices. She breathes in quick little pants as she lifts herself again, her tight hole sliding back up your shaft. As the head of your cock begins to pull against the tight opening from the inside she halts, not wanting to risk you falling out. She grasps your free cock and wiggles her hips, pressing the tip to her damp, currently unoccupied opening. She slides down and you enter this one easier. A gutteral moan escapes her lips.

Now that both of your cocks are sunk into her she seems more secure, smiling contentedly at you. She rests both of her hands on your chest and transfers some of her weight onto you as she begins to ride in earnest. Her moans soon fill the room as she slides up and down you, her speed increasing with each movement as the doctor fucks you against the chair.

She's working at a rapid pace now, her moans and pants a constant noise with the background of her skin slapping against yours. Her tits bounce with each thrust and you feel yourself getting close. "Doc, I'm gonna..."
"Not yet, I'm so close, not yet!" She orders and you do your best to obey. You grasp the chair underneath you and your fingers squeeze tightly as you try not to cum. Even as every motion of her body begs you to. Her thrusts take on a slightly different pattern and her moans more ragged and you squeeze tightly. Just a little longer... Suddenly she lets out a ragged gasp and as her pussy twitches around you, holding on is no longer an option. Both of your cocks start to unload deep into Dr. Black as she collapses against you. Her hands curling to grasp at your flesh as she shudders with her own climax. As the sensation of your double orgasm rocks through your body you feel like you almost black out for a second, the sensation so overwhelming it feels like your heart is beating ourside of your chest.

Dr. Black catches her breath, remaining laid against your chest. Her breasts pressed against you. She tilts her head to look up at you and she smiles, flushed with exertion. "That was good..." her fingers trail over the skin of your chest, drawing complex patterns. "If you would do me a favour and uh, not mention basically anything from the lube part onwards in any of the surveys you'll have to do? We'll definitely want to get you back for further study..."
You smile and nod, seems like a fair trade.

She leans forward and places a gentle kiss on your lips, more of thanks than romance. As she moves to pull off of you she suddenly gasps. You feel... something. She calls out to the dictaphone on the desk. "Second phallus receeding at 36 minutes after treatment." Professional again even as her pussy squeezes around your cock. She grins at you, seeming to enjoy the strange sensation as you retract out of her and her ass is emptied. Your desposited seed runs down her leg onto you.
"Was that recording the whole time?"
She nods, "but don't worry it's just for my own notes, I submit my records in writing."
She gently lifts herself, releasing your one and only cock and more of your cum. She hums contentedly as she goes to the desk and grabs some paper towels which she throws to you and you use to clean yourself up, gently probing the area where no penis now is to confirm everything is back to normal.

She nods to your clothes, "please get dressed. We are all done for today." Professional again, naked and covered in your cum and still a little flushed from her own climax but professional. You pull your clothes back on in slience but look back to her when you're done. She seems to be collecting some of your cum in specimen vials from her legs. Which you guess is one way to get study samples. You feel like you should say something.
"I, uh, thank you?"
She grins at you. "I am very fond of my job. Please do come back for the follow up study when we contact you."
To be honest you think you're still a little shocked, you nod and make your way through the door. You look back as the door closes to see Dr. Black licking some of your cum from her fingers. This was a weird research study...