Well, if you are going to go to Lithiums, you apparently already have one. The Lithium sales people recommend you buy those which will be used in series at the same or closest to the same time, just like lead-acids. Adding your electronics directly to one of the batteries could be detrimental to the batteries used in series not to mention it could cause interference on your electronics when you use your T/M.
I have never seen this "Bandit voltage splitter/isolator" and cannot find a good reference on my internet, so I'm totally ignorant as to its function. I did go to Monster Marine and saw a $900 unit that appears to be a "charge on the run" item.
Since you already have one of that brand purchased, the best time to pair it up has passed. Maybe just leave the older battery dedicated to your electronics and buy two new ones for trolling would be a good choice.
I don't know if Monster is a BBC sponsor. That could make a difference.