What Scoop said. I pulled my primary battery waaaay down trying to winch a guy out of a snowdrift, before I had the dual batteries. I was afraid to shut the buggy off, thinking it might not have the voltage left to restart. Now I'll be able to see how far down the aux (winch) battery gets without compromising the start battery, the dual VMs will show me that difference.
I have a winch (had one on all my SxS & ATVs over the years). I always keep the engine running during use for that reason. I don't have a stereo or radio, no big chase lights or whips (when it's dark, I sit at camp with a cocktail). I don't think I'll ever need a 2nd battery but I understand the reason why a lot of you do and the dual VM makes sense. I've got a single VM with a USB that does what I need. Thanks for the added details.
There can/will be times where you can't keep the engine running while winching. Also, the winch will draw far faster than charging system can put back, so keeping in the engine running during use only goes so far.
