I am not sure if the problem you talk about is that mechanically, the two levers are diffcult to keep in the same position of if the levers move together OK but the engines react differently. I suspect it is the fist issue. The adjustment screws change the 'drag' or force required to move the levers as wells as helping to keep the levers in the desired position. Too loose and the levers won't stay in position, too tight and the levers are difficult to move.
In addition to the drag, the control cable at the engine should be adjusted so that each engine revs the same ( or close to it) for a given helm throttle position. Another useful piece of info would be if this has always been a problem or if it is something new. It is possible that a cable is going bad or that the engine throttle linkage is binding or out of adjustment.