Your fuel tanks should not be connected to each other if factory installed. You should have 2 three way valves that allows your motors to either both draw fuel from one tank, or each engine drawing from one specific tank all the time. Maybe your fuel gauge is off ,you would have to top off both tanks and refill to the top for a few trips to accurately figure out if one engine is actually using more fuel. Yes as posted your fuel burn at cruise with optimum trim,calm conditions and light fuel ,gear etc. load should be 1.2 to 1.5 max. Not knowing any specifics ,like actual length, beam, health of engines, props, weight of boat and contents makes it hard to pinpoint your fuel burn