Cas and Jimmy don't even realize that somehow, something has gone supernaturally awry and they've switched bodies. Because who would even think of that, right? They got a little drunk, a little celebratory, and they wake up in a naked pile on the couch without the slightest idea anything's different.
Cas-in-Jimmy goes to Cas's room and puts on Cas's clothes. Jimmy-in-Cas goes to Jimmy's room and puts on Jimmy's clothes. They go about their day as normal. Then Dean comes over, takes one look at his boyfriends, and says, "What's with the switcheroo?"
"What?" Jimmy asks.
Dean gestures between the two of them. "You're wearing Jimmy's clothes, and he's wearing yours."
"No," Cas says slowly. "I'm not Jimmy, and I'm wearing my clothes."
Dean actually looks annoyed. "Come on, guys. I thought we were past the testing-Dean's-twin-recognition part of this relationship."
"And I thought we were past the Dean-failing-to-recognize-that-his-lovers-are-different-people part of this relationship," Cas shoots back, his tone matching Dean's precisely. He's always been quicker to anger than Jimmy, especially when it comes to the two of them being treated as interchangeable. He got into more than a few fights in school because of it.
"See, that's exactly what I'm talking about." Dean's fully engaged now, taking a few steps towards Cas and jabbing a grumpy finger at him. "You're trying to act like Cas, but I fuckin' know you, Jimmy."
"Apparently not," Jimmy grumbles. He's starting to get irritated, too. He doesn't take being mistaken for his twin personally, as a general rule, but it's true that Dean hasn't misidentified them in months. And his insistence that they're messing with him, instead of acknowledging and apologizing for his mistake, is quickly moving from 'obnoxious' territory into 'we've dumped people for less.'
Dean stares from Jimmy to Cas, then back to Jimmy. "Seriously, guys? All right, I guess we're doing this." He gets right in Cas's space and touches the corner of his eye, gentle despite his frustration. "Jimmy. Your crow's feet aren't as deep as Cas's, and you have that little scar…" He traces down Cas's cheek to the nearly invisible line just at the edge of his jaw, where Jimmy split his chin in a childhood biking accident. Cas doesn't have that scar, and yet there it is.
"Holy shit," Jimmy breathes. The other two men turn to him and he takes the opportunity to study Cas closely, which he hadn't had any reason to do before. It's different than seeing himself in the mirror, and different than seeing Cas. He's looking at himself from the outside and it's deeply unsettling. His face is scrutinizing him right back.
"Oh," Cas says with Jimmy's mouth, "fuck."
"I can't believe you guys didn't notice you weren't in your own bodies." Dean's very clearly trying not to laugh, a little amusement and a lot of hysteria. "Also, what the fuck?"
"I have no idea," Jimmy starts, but he's cut off when Cas, out of nowhere, grabs the front of his shirt and hauls him in for a kiss. It's deep and fierce, Cas's—Jimmy's?—tongue claiming Jimmy's (Cas's own) mouth as a hand slides over to pinch and roll at his nipple. Jimmy's knees buckle, but Cas holds him up, keeps on kissing and toying with him until Jimmy's so breathless his vision greys out a little.
When he pulls away, the smug look on the face he's borrowed somehow from Jimmy is 100% Cas.
"You're in my body," Cas growls, and it's possessive but not angry. "When I fuck you, I'm going to know all the tricks and all the spots that will drive you out of your mind with pleasure. I'll read every shiver and whimper and know exactly what you're feeling."
"Oh fuck," Jimmy says again, though it's a very different sort of exhalation this time.
"Fuck yes," Dean says, reaching down to unbutton his fly where he's already starting to harden. Swapping bodies may be the weirdest fucking thing that's ever happened to the Novak twins, and they may not know how or why or what they need to do to get back to themselves, but they can figure it out later. And maybe figure out a way to do it again, at will, because it turns out to be pretty fucking amazing, too.