Personally I think partners in crime sounds better. Though I guess it’s not really crime...

[Are they technically sent by the state?? Or?? Whatever, he just wants to avoid the problematic flag of best friends going alone on a mission of shadowy proportions.

Partners in slime? No, that works better back home.]

Partners in, uh, sneak. Sure.

[He holds up his hand in an upside down V sign, apparently as concession to the secret handshake.]