If you feed the winch with a big fat cable, then it needs a big fat cable to return to ground at the battery. Counting on a stock ground point might be OK if you check it for corrosion, etc, but how it gets back to the battery from there should be as short as possible.
I went with big fat cable to everything -- twice. So I have dual positive feeds, switched thru a Blue Sea switch. Then there are dual grounds coming off the winch ground directly back to each battery.
Corey,
I've got a Hellroaring system and they advise the same thing. Works great, as I have a CPAP machine and have sometimes needed to use my backup to start in the morning when I used it on 110V thru the inverter. Now that I have the correct 12V plug for it, I'm hoping that doesn't draw as much and won't need the backup.
That said, I also have my backup battery wired to feed the winch thru the Blue Sea switch. True, it does compromise the backup nature of the system -- IF you happen to throw the switch and use the winch. Until then, your backup is totally safe against any draw.
And most of the time when you might need both batteries, it's usually not an issue with starting, but with getting enough juice to the winch. Unless you blow up both at once, you should have enough left in one to start a warm engine unless you're going for total meltdown:Wow1:
This is the first dual battery setup I've run and I've not had a need for the winch yet. But the last time I had a Land Cruiser with a winch, there was a time or two when a second winch battery would have been nice.
YMMV