Thanks Snips, yeah you're spot on, and pretty much answered all the questions I have!
Unfortunately when I ordered the boat I had had a good look at the same model that a fishing charter has here.... except that they've only got one outboard! Never thought about an extension to put the two motors on, and purchased the TM260 to get some good deep resolution, only to find out later there was no where to install it! The boatbuilder ended up putting it inside the hull next to the batteries, right on the "V" of the hull (see photo).
It actually works quite well, very little apparent interference and doesn't miss a beat trawling. The big problem is that cuts out after only 115m, and every now and then it cuts out for seemingly no reason in much shallower water. Obviously there are issues with Accu-fish and bottom descrim, but I haven't used them much until now.
I guess the only question left is: if I get myself a more appropriate transducer (thru-hull), can the TM260 been cleaned up nice enough to sell after being installed inside the hull?
Cheers,
Platta