Yes and No!
In theory it will work, in practice there are problems.
The battery you add will want to be a match to your present battery including actual capacity.
If the present battery is 3 years old, you will not want to add a brand new battery that is in much better shape than the old one.
It is preferred that both will be matched Deep Discharge Batteries.
If one is attached to the Sonar and Radio it will be drained faster than the other.
If one is attached to a motor, it will be the only one that gets charged.
If the two batteries are not at the same level of charge the fuller battery will overpower the lesser battery.
This can all be done as long as you keep a voltmeter on both batteries and constantly monitor the imbalance.
Do not let either battery get too much below 12v or it will suffer badly.
Your best bet is to get a matched set of Deep Cell Batteries to run the Troller, and charge them as a pair from shore power.
Move all your minor house loads to the starting battery.
Sonar and minor items will take 10-20 hours to deplete that battery.
A battery switch to parallel one of the deep cells with the starter is cheap insurance in the event that you ever kill the main start battery.
Otherwise if the 8hp Kicker has pull start,
It will get you headed for home and put enough charge the the start battery that after 20-30 minutes it should crank the main engine.
I would jury rig the Troller before you invest in new batteries.
The 8hp gas kicker engine may already perform more to your liking, and be a lot less hassle since you already have it installed.
The cost of a set of batteries may buy more than a few years worth of gas for that kicker.
