Tommyboyrn,
I just added an SI unit to my DH5 unit on a pontoon boat , I had to make modifications to the bracket so I could keep the reg transducer and si unit between 6-12 inches together but I called Lowrance and the SI unit is a 70 degree unit so that means that you need a 15 degree clear space on both sides of the unit but with my pontoons setting pretty low I had the 15 degree clearance and so far have not had a problem with blocking the signal.Also you only need the suggested 6-12" spacing for good overlay anything past that like tranducer one one side and SI on the othe side of transom will put the overlay off by that 4 or 5 foot..
Gary