definitely an interesting idea...
I don't think powering both axles will cause any binding. OTR tandem axle trucks power both axles through a common drive shaft and they don't have any issues. The tandem axles turn on a similar arc so there shouldn't be any windup in the drive line.
I am curious about two potential issues though:
How are you locating the secondary axle? Is it by a 4 link with air bags? It almost appears you have a 4 link suspension, however the links are pivoting from the rear of the frame? Have you thought about how this will function under load? Anti-squat comes to mind with the positioning of the links.
Second, if you are planning on using the secondary axle brakes, how will you tie those into the existing hydraulics? Have you come up with a master cylinder that has the capacity to move two additional wheel cylinders/calipers?
Finally, the drive shaft routing from the t-case will get interesting. It seems to me that it's going to take a lot of thought in the Ujoint phasing and angles to snake around the middle axle, and keep all the joints in companionship angles so that they cancel each others vibrations. There's always the CV route, but I don't think there's one in existence that can handle the torque of cummins.
Food for thought....
