They should go in parrellel and remain a 12 volt supply, just 2x the Amp hours.
Keep the connections as short as possible and as large a gauge wire possible.
NormAlly, you would establish 2 batteries at the same time, and matching in make and size.
They need to be the same AH size, unless you intend to operate and charge each battery independently , or the stronger of the two will be become parasitic to the smaller.
Jim & Kim from Colorado
2014 Eagle 30.5 RLS
2015 Dodge 3500HD SRW 6.7L
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