Put the second battery in just for peace of mind and stagger their age. This way you won't have to replace two at once, you can do 1 at a time during the necessary seasons for replacement.
It doesnt matter if 1 batt is 20 yrs old and the 2nd is brand new, they are totally seperated in the system.Of course if you dont run and isolator or combiner switch, thats a whole other ball of wax.
What the biggest "juice hog" for electronics? I only run my small fish finder and STS radio when I'm out. I switch between batts (switch) during the ride out and ride in. Neve noticed much drain (and I've had some hard starts when the engine is warmed up).
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