No worries about the transom mount, I've learned the evil of my thoughts. Mike, I'll take a look at that today. If flat, then a single mount is all I need and I'll just pick up the one. Thanks!
So I had to go back down to the boat and see this flat space. Yes, there are two <5 degree rise flattish spaces in front of each propeller and on either side of that huge keel. It drops down about a foot lower than the mountable areas and with the transducer of choice only being 6" tall, it can't see over the keel to the other side to give a true side view so another's gonna be needed. Convenient that they come packaged that way, even includes the y and extension cables needed to run them in tandem. Bloody expensive, but her electronics are a bit dated.