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Isolator is nice if you are going to draw one batt down, but otherwise not necessary to charge both batteries.
Been running multiple bronc's for 25 years, dual batt's, and no isolator.

In fact, I even run dual 3G's on my rock crawler (second one has switch on field) and even they work without any
fancy circuits to ensure both will charge at the same time.

 

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Dual 1000 cca batteries simply wired in parallel View attachment 209438
in my '78. Leece-Neville 165A ambulance alternator. Keeps up with my Warn 8274 and a 2000W inverter.... View attachment 209437

Wiring in second pic is so clean, you can't even tell it is there.