GM offered it once as well, though I haven't seen it on the order sheet for about a month or so now.
The dual alts would be for extra power. For example some folks have a plow. The hydro pumps take power to work. Extra accessories that take power, like DVD players, subwoofer stereos that are becoming more common from the factory take larger amps. I would suspect those might be just a few reasons why dual alts are offered. I know that for the price vs. the power generated, I would be inclined to just do it.