For what it's worth Cuppa, I've hooked my dual battery solenoid up to my winch switch in a way that disconnects the dual batteries when the winch is activated. Given my winch can draw 500A at max draw, and my isolater is only rated to 150a, it just wasn't going to cut it. Granted, we should only be seeing 250a to each battery when connected in a winch scenario. Given that most people that I've seen with dual batteries tend to have the second battery as an AUX deep cycle type battery, they don't tend to deal well with big current draw the same way a normal crank battery will.

My crank battery is 800cca or thereabouts, and it dragged my patrol with a navara on the back, single line up a hill for about 30 minutes, with it's only breaks being to setup the next tree/winch. About 300m of winching 2 vehicles at the same time up a big ass hill with 0 traction (granted, I wasn't stuck, just too slippery to get up the hill) and the battery handled it fine. Gave it a good charge when I got home on the trickle charger but all seems well.