I’m a bit late to the party I see Matt.
Higgy has pretty much got it. The Collie could start quite happily in one battery but (and this is pretty much theory as it’s unlikely) if it took a lot of cranking to start, there would be less reserve available. In other words if you left your lights on and with two batteries it lasted say 2 hours, it would probably last 1 hour before the lights went out.
As for one battery being used for starting it would depend how it’s all wired but broadly speaking I would say both batteries are going to supply starting current in almost equal measure. ‘Almost’ due to internal battery resistance, clamp resistance and cable resistance plus points of connection of the starter cable and earth. The difference will be small but it will be there.
Can it start on one battery? Yup. If you wanted to, you could remove the second battery, alter the wiring to include a split charge relay or better still a DCDC charger and have the second battery as a leisure (deep discharge) battery that won’t discharge your starter battery with all those lights etc.
Again, you don’t have to add a leisure battery setup, but doing so separates your ‘house loads’ from your vehicle loads and starting. The advantages of this are that you can use a deep discharge (leisure) battery which is designed to take a greater discharge than a starter battery plus if you do discharge it, you’re not relying on it to start the truck.
You don’t need a third battery. Ideally uprate the battery you intend as your starter battery to the biggest Ah and CCA rating you can physically fit in the space, then use the space the other battery occupied for as large a leisure battery you can fit in there and feed one from the other via a charging system of your choice. No reason not to use a split charge relay like the Durite as it’s so close and under the bonnet, with a lead acid standard construction (vented) battery.
Winch use would benefit from a solenoid linking the two batteries to work as one when winching. Whether to draw from your leisure battery or the main starter batter has many answers, most or them will be right as there are pros and cons to each. Just consider that generally, winching is infrequent but performed with engine running to charge the system and generally power the winch.