Hello Malcolm,
It sounds like you have two problems. First, you are drawing from two fuel tanks but only returning to one. And the "balance" tube is not doing what it is supposed to do.
The fuel pump on the 2 stroke Detroit's will move 2-3 times as much fuel as the engine will use, the excess fuel used to cool the injectors, and return to tank. There is a restictor fitting on the end of the fuel rail that keeps pressure between 60-75psi, after that fitting the fuel is supposed to return to the tank it is drawn off. For some reason, both engines are returning to the same side of the fuel system. This has to be a plumbing issue, unless you folks in Oshawa have discovered a way to have a Detroit manufacture fuel!
Personally, I would only use the upper sponson fuel tanks to feed the engines. This keeps the fuel feed line above the fuel pumps. Also, typically there is a primary fuel filter between the tank and fuel pump, and a secondary filter between the pump and injectors.
On a lighter side, I once worked with an old timer who kept calling the Detroit diesels "converters". I asked him why, and he told me because "they convert fuel to noise".
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