Pumps in parallel? are you sure? I am thinking series, one ahead of the other? but why? Just use one or the other! not two. Remember, if you have crap in the tank, you have crap in the gas, in the pump, and in the carburetor and it shows in the filter if you have one installed. Most old gas tanks will deteriorate and give up their galvanized surface after say 20-30 years. Cleaning and treating tanks that have started to deteriorate is a waste of time and money. Take several fuel samples out of the bottom of the tank at the fuel line using a clear glass jar. Inspect the sample for rust flakes, black specks (galvanized material), dirt, or other crap. Fuel should be clean, clear with only a slight color with NO TRASH or water (which will settle at the bottom of the glass jar). If anything like this is present, replace the tank, clear the fuel lines, change fuel filter, and clean the pump, and carburetor fuel bowl and needle valve and seat. I would suspect you have a FUEL TRASH PROBLEM caused by a deterioated tank.

Best Regards
Will