Found it in the SRD.
I feel like the ammo part isn't an issue, simply because the Gunslinger already can make his own ammo easy enough as long as he has the kit. (per rules, at least). It was more the style factor at that point. 2 guns is an impossibility without revolvers or alch cartridges (admittedly I could take the skill set to make those, I suppose).
Once we're going with a crossbow, I could just as easily go with a rifle or some such, no? Though if I'm doing that, style factor does think I might enjoy a crossbow more. Ah, vanity.