If the M-16 is still available when Wallace gets there. He should be able to get his own rifle back from Samantha if he wants it, though. If Stardreams joins us by that time, it's up to her, but if Samantha's still an NPC I think she'd give it back without the need for a roll. Of course, the rifle is even deadlier in her hands.
The .45 pistols are also all taken. Sally, Luke, Bob Larkin, and Lynne each have one. Bob and Lynne both have two pistols currently. Wallace's .357 Magnum is pretty decent, though, and there are extra bullets for it.
Dual-wielding pistols is possible, but very difficult. Anything larger is right out. You wouldn't be able to fire them at separate targets simultaneously, though (this isn't a John Woo movie). You could fire them at the same target at the same time, but these would be unaimed shots and thus at 1/5 skill unless you have both a high enough Handgun skill (which Wallace does) and laser sights (which he doesn't).
You can also fire only one pistol at a time with no penalty, which means you can go longer without having to reload (the "New York reload").
I think the Spot Rules for combat on page 2 of the rules thread have some information on dual-wielding and unaimed shots.