It will pull from both until the primary battery drops below 12.6. I've seen my voltage gauge drop to 10.5 while winching. My gauge is connected to the aux battery so it was the one that dropped to 10.5 and my starting battery stayed at 12.6. That's probably very important because of all the electronics on these machines, they all pull from the primary battery.
Just give me a day or two heads up before you want to come out. I'm home today and tomorrow but I'll be in North Carolina Monday through Wednesday.