Parallel with no isolator\solenoid.
Positive to positive,and negative to negative.Both negatives can be grounded to the engine\chassis,they don't have to be connected together,as long as it's a good ground.This is what you want for a plow setup,as it gives you double the reserve capacity.
An isolator serves two purposes.
1- It separates the two batteries,so that a heavy draw on one battery will not discharge the other.This is better suited to a camper or motorhome,where you may run one battery dead powering the fridge\lights,etc,but still need the other battery to start the vehicle.While this would be good on a plow truck to prevent the starting battery from going dead,it gives you no increase in reserve capacity,so the plow will run the aux battery down faster,just like running the truck with one battery.
2- It also regulates charge rate to each battery based on the individual state of each battery.So if one battery is dead,and one is fully charged,it will only charge the dead one,and not overcharge the fully charged battery.It will usually charge the starting battery first,then the aux battery.This is again suited to an application where the batteries are discharged at different rates.If you did this with a plow truck,your aux battery for the plow wouldn't see enough charge rate to keep it fully charged.
That's where the problem arises with the above mentioned solenoid,and using a switch to selectivly activate it.The batteries could be discharged at differrent rates (switch off),but will be charged at the same rate (switch on),resulting in possible damage.We used to see this setup on a lot of older motorhomes and campers.It would allow you to park,and disconnect the starting battery so you wouldn't run it dead,yet still be able to power the rest of the camper with the other battery.It usually resulted in very short alt and battery life,and even a few blown up batteries.
I have tried many different setups with my plow trucks,and two BIG batteries in parallel,with a big alternator is the way to go.No switches,no relays,no isolators to break down.
Changing the batteries out every few years is a big help too,as the tend to get weak from the constant heavy discharge\charge rates created when plowing.Leaving them in too long will damage the alternator over time.Don't use a deep cycle battery either,as this is not really a deep cycle application.
