The problem that you will run into is that each alternator has it's own regulator and no matter what you do, they will never want to regulate to EXACTLY the same voltage. Because of that, one will always be working harder than the other. There are at least two ways to overcome this.
1) Run the two alternators on seperate circuits, so that one is just like it is now and the second one charges only your axuilary battery. Then wire your winch and maybe off road lights, etc., only to the secondary circuit. Now you may want to run a solenoid and/or battery isolator between the two circuits. A battery isolator is just a diode so with good current through it, it will drop more than a volt. This could be enough protection that you could use it to let the primary circuit support the secondary when winching. If your secondary battery is starting to get low, the primary is always more than a volt higher than that. Or you could let the solenoid select between the batteries for starting or hook them together. It all depends on what you want to do.
2) Run identical alternators from a single voltage regulator. This will get almost the same current from both units. However, a regular regulator may not be able to support all of the field current for both machines, so you might need a bigger regulator then the one normally used.
I was actually thinking about dual alts before, but I was dreaming up a way to use and A/C clutch on the secondary with a relay to kick it on only when the voltage gets below a certain threshold. That would minimize extra drag on the engine when it isn't used.
<font color=green>There's nothing like the smell of a rich V-8 in the morning...</font color=green>