So presumably these 2 batterys will need wiring in parallel to increase amperage but still be 12v?.
Spot on
No, no, no, no and no. And just in case you're still unsure, NO.
Sorry to shout but for Gods sake don't wire batteries in parallel unless they are brand new and exactly the same. Especially if you have them wired up for different circuits (like one for the winch and one for cranking the engine).
Why? Because if there is any imbalance between the batteries, for instance, if they are different makes or different ages or differently discharged, then the one with the greater capacity will discharge into the one that has lesser capacity very quickly and with potentially VERY dangerous results. Possibly even exploding!
Look
here for a far better explanation of the problem and a good, simple and effective (if not ugly) solution.
You may also want to consider what type of battery you use. Some folks use normal "car" batteries that are designed for cranking the engine whilst others use Marine or Deep Cycle batteries. The Optima range of batteries are very very good but very very expensive too. It's probably best to avoid the Marine type as they are designed for deep discharge which sounds ideal for winching, but the problem is that most winches will suck vast amounts of current when under full load (which a good reason to buy a bigger winch than you need so it's under less load). Big currents are bad for Marine batteries because they are designed to supply low currents for hours and hours on end, like lights, not high bursts of current for comparatively short times, like winching. However, most car batteries are designed for starting engines which big current draw for a short time so they are better, but get one that is as big (current burst wise) as your winch needs - most 9000lb winches will suck about 450A, if not more, at full load on first layer.
