I can only say this about the Allman Brothers, not sure about the other bands that had two drummers.
Very close friends of mine who moved away a few years ago had started a new band, doing "jam band" stuff (allmans, dead, etc.). They were trying to get me to come out to visit them and we talked about possibly having me join as the "second" drummer. I thought at first it was awesome and wondered why the need? So I made the trip with a drumkit and spent an entire weekend with them at their rehearsal place.
We were working on "Statesboro Blues" by the Allmans. My friend who was the drummer, pointed out that there is two distinctly different parts, and he was right! Another song was "Jessica".
I never ended up joining the band (moving, job, etc.), but it was very interesting to hear and play that stuff.