Will not fill second Gas Tank

The routing of your fuel return hose from the carbs and breather hose from the filler neck to the right side fuel tank are correct. You can look at the factory parts manual to see what the fuel tanks and filler/breather hose routing is supposed to look like.
Your thoughts of a blocked fuel balance pipe or breather hose causing the R. tank not to fill are pretty much the only possible causes, except for the tank being welded shut. I would try disconnecting the breather hose at the top of the tank and blow through it to check for a blockage. If that's ok, then it is time to check the balance pipe/hoses for blockage.

 

While I don't have much experience on Dinos, I've encountered similar problem with two Daytonas and on both the culprit was the balance hose having been routed so that it became pinched shut by the LH trunk lid hinge when lid was closed.
Both still had original, dark green"cavis" hoses, likely installed by the factory (or Scaglietti ?) as both had suffered the symptoms for decades and hoses were almost petrified.
Once re-fueled full, the gauge would show "full", but even after couple of short drives or just sitting long periods, some or half the fuel would slowly seep into the RH tank and show "1/2" or so at the gauge.