If it were me, and it is not, I would move the older sonar to the front and mount the side imaging on the dash. First, I'd mainly use side imaging while searching. I assume we are talking bass fishing here.. But I'd still mount it so I could turn it and see it while up front. I only use a bow mount very sparingly when casting. I also wouldn't like the idea of the side imaging transducer turning as often as I turn the trolling motor. I'd never quite know what I was looking at. Best case would be to have two networked units where you could split a side imaging signal from the back with a TM mounted puck. But I get where you are.. You just want to make the best use of what you currently have.