You’re right-- it was Otis Redding! My bad.
From interview:
I remember one day down in Muscle Shoals, I figured I’d been there long enough, so I asked Duane Allman, "Man, tell me something, why do you want to have two drummers?" And he said, "Because Otis Redding and James Brown had two drummers." So I didn’t ask him anything else, I said something like, "Well, that’s cool, because I was one of the drummers with Otis."