As already mentioned, the batteries are wired in parallel, so they should share the load. I always run the engine when using the winch so as to avoid a heavy drain on my starter batteries. In the perfect world, I'd have a separate bank of isolated batteries for the winch and other accessories, but there's only so much room under the hood.
John (brussum), KL3JC
2002 Ford F350 Lariat, 4X4, 7.3L TDI, 2" lift, 35" GY MTRs, 4.10 gears, TJM bullbar and 12,000 lbs Warn winch, ARE shell, Yaesu FT-1900, external auto trans and coolant filter
2004 Ford Expedition Eddie Bauer 4x4 (the wife's family truckster)
2008 Toyota Land Cruiser, BajaRack, IBS dual battery system, Yaesu FT-8900
1990 M101A2 Expo Trailer (in the making)
IRLP node 7036, Alexandria