I've thought about this a few times. I think ultimately whoever owns the face in their Creator owns the design. But a collaborator could give credit to the "guest" artist. For example, let's say Tomaja Designs (@Tomas) provided you with artwork as a paired collaboration on your next Explorist #012 design. You might name the face Explorist 012 featuring Tomaja Designs or F/ Tomaja Designs or something like that.
The concept is certainly not new. Even all the way back to the 60's was not new.
"Under Pressure" featuring David Bowie
"Smooth" featuring Rob Thomas
"The Thrill Is Gone" (B.B. King) featuring Slash
"Walk This Way" (RUN-DMC) featuring Steven Tyler
"Crazy In Love" featuring Jay Z
"While My Guitar Gently Weeps" featuring Eric Clapton