I have a couple of Humminbirds that are networked together and through the options I can select which transducer I want to use, I never have used this feature except for playing around with it when I first got my boat.
That being said, it sounds as if your plan should work, I don't know what the Lowrance mounting brackets are like, but Humminbird makes quick connect brackets, so that it would be possible to have a bracket in back and one in front, you could then just pop the unit out and move it to where you wanted it. I'd think you'd probably need two transducers to make it work, but this isn't something I have researched so I don't know the details.
If you have time today, run over to the Classic Expo in Tulsa, Humminbird and Lowrance both are there, with some very knowledgeable staff on hand that could show you what you need to do.