Converting tandem to cargo bike
If you want to have a real cargo bike, e.g. an "inverse Bullit-style" the challenge will be structural integrity of your frame: you can't easily cut the 2nd diamond of your frame without risking to break the frame. 

However, I don't see a problem with removing your back saddle, cutting/removing the back cranks and building a sturdy plywood/mdf cargo hold left and right of the frame/using bags to attach. You could still haul a ton of stuff but would have the frame in the middle. In fact, there are quite some people that have done this before.