you are correct, one portion of the tank (larger portion is primary tank and smaller portion of tank is secondary. It has a small tank inside rail of frame for air compressor relief valves and governor psi. I do have a couple of nice small tanks off a late model RD that I could use, but back to your question, I’m not sure if it would supply the right amount of volume with out breaking out the calculator to do some un welcomed math.lol. I could pretty easily do the two tank mount and leave primary on drivers side and move secondary (I believe, maybe I have that reversed) to passenger side with a small amount of re routing lines.
It’s a single axle, the picture with the axle under the truck is the way it was when I bought the truck. It’s some kind of home made setup. The trailing bar set up was used with mack hangers. Which isn’t correct as mack hangers need an eyelid spring pack to be correct. The truck had the open hook on both ends of spring. This was causing drive line shift and vibrations and the trialing arm was some kinda home made deal. I’m going back with correct spring and pins and remove trailing arm all together.