[gslpro]

In most twin tank set ups, you have a three way valve for each tank. This allows port to port starboard to starboard, or only drawing off one tank for both engines, or combining tanks to draw off both tanks for both/either engine. The valve may be not all the way closed or open for each engine to properly draw. I believe (without being at the boat to look right now) that you should have two three way valves. Check to see if you have them, and their respective positions.