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.

 

The lower balance tube is about as unsophisticated as it comes so there is nothing short of a blockage that would keep it from doing its job. That said I would be very suspect of any blockage as the tube is of a very large relative diameter and blocking it would be near impossible unless something huge came loose in one of the tanks. Be sure to disconnect your battery when doing your investigative work as the slightest spark could be a disaster.

 

To prevent confusion, could you clarify what do do mean with “balancing tube” and other “vent lines”, etc ?
Perhaps a copy of breakdown images from parts manual, indicating each by their item (or part) numbers in said images ?

 

Glad you got it resolved, but to assist others, including future readers having similar problems & prevent confusion, clarification of the offending item would still be great.

By “the vent line” do you mean the circled item # 30 in this diagram (image copyright Ferrari/M.C.P.), for which I used a term “balance hose”, i.e. green “cavis” hose which allows air to escape/vent from the RH tank during fueling and maintain equal amount fuel in both LH & RH tanks at all times or something else ?

Also, if it is/was the culprit, I would recommend inspecting what was the cause of it not working properly.

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Thanks for clarifying & confirming my suspicion.
Your speculation/suspicion makes sense, especially if the balance hose/vent line, i.e. green “cavis” hose, is somehow “drooping” down somewhere between the filler neck & the RH tank and allows fuel to enter & stay in it during re-fueling and/or driving, essentially creating an air lock for the RH tank air venting.
Again, I would recommend checking &, if needed, correcting its routing path to prevent this becoming a reoccurring problem.