I would start by removing the existing transducer and mounting one of the HDSi transducers just to the right of that position. Hold off on the second HDSi transducer and the 2D transducer until you figure out if you really need them or not. If you need the 2D transducer for high speed readings than I would mount it on the opposite side of the motor in about the same location so as to lessen the amount of Si sonar that it blocks to that side.