Thanks for all the replies and helping me figure this out.
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Most likely this previous owner added the second at the time the winch was installed. Both batteries are connected from the positive terminals to the same point on the fuse relay center box, here's a pic:
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I don't see any battery isolator so I guess both batteries share the load and would be drained equally? I'm assuming I could pick up something like this so I can use the driver side battery to power the winch and other accessories without worry of draining the main passenger side battery:
http://www.amazon.com/Stinger-SGP38-80-AMP-Battery-Isolator/dp/B001HC6UJ0/ref=sr_1_5