dual batteries

Unless you want weight and or balance , two batteries is a bandaid.
A decent alternator should provide enough juice to run all your accessories with amps to spare.
Switching it off to give you more h.p. is cool if you are class racing or need every 100th you can get. I don't see it as necessary for a bracket deal.
We used a Tic-Toc sequential switch to cool down between rounds.......especially late in the day when time between passes got shorter.
Shogun industries made them..........they would cycle between the water pump and fan , and could cool the car down in about four minutes.
Something like 30 seconds of water pump to cycle the coolant out of the radiator , then 45 seconds of fan.
Even using this , starting was never a problem in the late rounds.
I have not tried the trolling motor battery , but I have heard of guys who liked them.
I always get the biggest battery I can and make sure everything is well grounded and works like it should. Electric water pump , fuel pump , fan , MSD ignition and line lock is the extent of amp load and I have a charging system rated at about 40% over the max load.