So what I understand we need both charger, one for charging batteries when you get back home and another one dc to dc for charging batteries with engine when it is running?

Most larger motors have an alternator (charging coil or stator are other names) and will recharge your battery - just like in your car/truck. If it doesn't have one, then the AC/DC charger is what you need for your starting battery.

Normally the alternator on your gas motor only charges the connected starting battery. Some companies have chargers on the market that can take that current from your alternator and power the dc to dc charger so that you can use that to charge several batteries not just your starting battery. Powerpole has one, I think its called Powerpole Charge, and MillerTech has one too I think the model is the Darksleeper.

You really only need that capability if you don’t have enough capacity in your deep cycle batteries to get through a full day on the water, or if you find yourself on a multi day trip with no access to AC shore power.

Most of us only need a multi bank onboard AC to DC charger to plug in once you’re off the water.

If you get good quality deep cycle trolling motor batteries with a high reserve capacity they will last all day and then some. You won’t have to worry about charging them while you’re on the water.

My particular situation requires that I often charge the starting battery when I get home due to fishing electric only lakes.

I have a 2 bank onboard charger for the 24v trolling motor batteries and a single bank onboard charger for the starting battery which runs 2 graphs, livewell, bilge and starts the engine.

Never an issue on the water.