As I understand it ... hooking up two batteries together that are different series, &/or age, is not a good idea.

I would go ahead & buy the two new TM/deep cycle batteries (as large a series as will fit) and the connecting cable.
Check the power cord of the two depth finders and you may find several feet of excess cord rolled up & bound ... if so, I'd unbind that cord and move the connecting ends over to the starting battery. (I'm assuming, of course, that you're thinking you have to run a "separate cable from my start battery to my finders" because the power cables wouldn't reach the start battery). But, if your finders aren't getting interference from being hooked to the TM battery, now ... then there may not be any reason to move them to the start battery anyway.

It might help to know : boat length/style - trolling motor # thrust - current TM battery size (series) ... as well as what is the max speed you can currently maintain. Parallel battery hookup will give you more "time", but not more "speed".

I'm no expert in any of this, so I defer/refer you to those that are more experienced & knowledgeable on the subject. Maybe CatFan will chime in !!