In the multiplayer mode of Call of Duty: Modern Warfare, you can only use one primary weapon and one secondary weapon by default. This is fine if you use an assault rifle or something like that as your primary and don't really need a secondary weapon, but it can be super annoying if you want to use a sniper, and have a more powerful backup than just a pistol.
So, here's how you can use two primary weapons in the same loadout: All you need to do is to equip the perk called "Overkill". This Perk will allow you to choose two primary weapons in your loadout, but this also means that you cannot use a secondary weapon. It's a nice Perk to use if you want to play two different play styles in the same match.
Unlocking the Perk takes a lot of time though, since you need to be level 38 to get the Overkill Perk. It's definitely worth grinding towards, because it can be very impactful if used correctly.