Joining batteries with a CTEK 250s DUAL system

[PeeBee]

Thanks for the info mate. The idea was to only use the house battery in an emergency if the crank battery is not viable. How about if I put another isolator between crank battery and winch so then the winch can draw separately from either rather than both? And if so, would the winch earth need modifying?

Winch earthing will be fine, its common to the chassis I am imagining at some point. Are the batteries the same, ie both crank or crank for start and deep cycle for house? Deep cycles dont cope with high current drawdown at all. A few guys simply run the winch supply off the crank and make sure they dont turn the vehicle off prematurely or before the voltage level has recovered. If you go the isolator, that overcomes the issue you raised around 'unviable', as linking a good to an unviable will just generate two unviables! I think your isolator should be like a Blueseas battery selector switch and then you can select both batteries, crank battery or house battery independently. they have good amp ratings for the duty rating. Something like this
https://www.ebay.com.au/itm/22267249...3ABFBMuO39it1i