Assuming you now have one battery for the outboard and one for the trolling motor (and "House" loads), you'd need a 3rd battery, but its a bad idea to put a new battery in series (or parallel) with an old one, so you might end up buying 2 new batteries.
then you'd need a charger set up for the 3 batteries. You probably already have a 2 battery charger in the boat. Most likely that can be used for the trolling motors then just use a single battery charger for the outboard starter battery.
There are some 12/24 v rated trolling motors. You may or may not have one. If not, then you need a trolling motor that is made to handle 24 v.
Usually the trolling motor power is split from all the rest of the electric system. But someone may have changed that.... So you need to ensure you are not linking the 24v to the 12v systems.
