Noun: An appropriate rhythm while stitch-fucking. Two males are double penetrating a woman in her anus and vagina, and as one goes in, the other is going out. This appropriate rhythm ensures that the woman is fully penetrated at all times and does not die or get split in half. This also conveniently prevents the two men's testicles from touching each other, which could call their heterosexuality into question.
Matt: Man, when I grow up, I want to be just like Josh and Stacy. They're masters of synchronized stitch. All the chicks are talking about them.
Zack: Yeah, I hear their balls don't even touch. They're gonna turn synchronized stitch into an Olympic sport!
Noun or Verb. Vaginal and anal double penetration: Occurs when a woman is being penetrated by two men - one in the vagina and one in the anus, giving the appearance or feeling of being stitched by a sewing machine.
Jeff: Hey, man, where's your wife? I thought she'd be at the party.
Ben: She's probably off stitch fucking Stacy and Josh again.
Jeff: Jeez, again? I thought she did that this morning.
Ben: Yep, she sure loves her some stitch fucking.