Smiling you don't turn the dial and start to pump, filling Sandy's ass with water as it inflated slowly. Sandy giggled with the hose in her mouth as her ass continued to grow to match your own in size. Soon she was sitting on a large ass like you were, soft and watery cushions as she pulled the hose out of her mouth.
"Hey, stand up and turn around." She said, getting up herself as you followed her, turning around as Sandy turned her head and looked at you before squishing her ass against yours. The feeling was pleasurable as you felt the nice squishing sensation before you and Sandy both pulled away and sat down again.
"There is a second option. You can deflate something as well but sucking air out." Sandy said as she turned the dial and handed the hose to you. "What are you going to do?" You asked as Sandy smiled. "Make us match, one part at a time." She said as you shrugged and put the hose in your mouth as Sandy started pumping, sucking air out of... something. Looking down at your dick you watch with awe as it starts to shrink, shriving up before being sucked back into you along with your balls. But Sandy kept sucking more air out of you with the pump as your crotch started to form a cavity, your new vagina as you moaned with the tube in your mouth, Sandy pulling it out after your vagina had fully formed. Now you and Sandy matched in gender and Ass size. "My turn." Sandy said as she stuck the hose in her mouth and you looked at the dial. What to do?