It's always worth checking tachos against a digital tacho direct from the crank pulley, they are dirt cheap to buy and even if inaccurate at least it will be the same for each engine (they are usually spot on), once confirmed both tachos are ok maybe set the throttles together at normal river speed, idle should be ok as it's not the cables that set that, they will still be out at other throttle settings but closer in your normal range.
Mine are never in the same spot, one engine always uses more fuel, you just get used to it, I turned my helm gear by 180 degrees as one had a worn spot at sea cruising speed so would keep creeping back slightly, I figured they wouldn't be doing those revs for long periods in reverse so shouldn't have the same worn spot.