You can write the credit for the other person in the caption, but there is no current way to credit more than one person. This is a very requested suggestion, though!
I recently uploaded a picture for one of my characters that was collab with another artist in which we both worked on the same character (He sketched, I did the linework, and so on) but I can only credit one person for it!!
Heyyyy I had the same idea!! My thread is HERE.