Yes, you can probably dual-wield guns, as long as whatever modern rules you are using (d20 Modern, Spycraft, CoC d20, etc.) doesn't specifically prohibit it. However, TWF only applies to melee weapons, so you'd only benefit from Ambidexterity.
Okay, after a little math, here’s what I came up with. I’ll use the system presented in Call of Cthulhu d20, since it’s the only one that I have nearby. Since firearms aren’t given sizes, I’ll assume that a pistol is Small, and therefore is considered a Light weapon, while shotguns and most rifles are Medium. So…
When dual-wielding pistols, it follows the same mechanic as D&D, where Ambidexterity affects it, but Two-Weapon Fighting doesn't, giving you a -4/-4 to hit. (Assuming that pistols are Light.) You can also use the pistols' multiple attack ability to fire each pistol multiple times. An investigator with Ambidexterity and Multishot firing two Light, Standard pistols simultaneously has a total of –8/-8 –8/-8 penalty on his attacks. This is –4 for dual-wielding with Ambidexterity and a Light weapon, plus –4 from firing a Standard gun with Multishot.
I assume that an investigator can also fire a shotgun or rifle with one hand. (I realize this isn’t exactly realistic… Please ignore realism for a moment.

) This would probably use the rule listed for Heavy Crossbows in the PHB, where a PC can fire one one-handed, but they receive an additional –4 to hit, on top of dual-wielding penalties. If so, you could fire two shotguns or rifles at once, with the total penalties being –10 and –14. (Without Ambidexterity, and with shotguns and rifles being considered Medium. It’s –10 and –10 with Ambidexterity.) This is –6 and –10 for normal dual-wielding, plus an additional –4 to hit from the crossbow mechanic. You are also able to use these weapons’ additional attacks if dual-wielding. (Assuming the gun in question isn’t pump- or bolt-action.) An investigator with Ambidexterity and Multishot, dual-wielding two Autofire rifles, could fire both simultaneously for a total of –14/-14/-14 -14/-14/-14 penalties to his attacks. This is –6 for dual-wielding two Medium weapons with Ambidexterity, plus –4 for Autofiring with Multishot, plus –4 for the crossbow mechanic.