It's been stated by Chad in a number of interviews and even clips from studio sessions (ie. Making of Justified album) that P usually messes with the drums on the beat, and Chad usually the one on the melody. Like Chad corrects all of P's work. Go listen to the first version of Justin Timberlake's "Last Night" beat that P made and see how Chad fixed up the joint and made it sound so damn smooth and what he did on the bridge of Last Night.
Their roles vary (as stated by Chad). The majority of that studio footage is of both of them reworking demo versions of the "Justified"tracks. Judging by that footage, there is no way of knowing that Chad was the one that created the bridge and "fixed up" "Last Night".