If you are new to Call of Duty: Modern Warfare, then this is something you should definitely consider and enable. If you want to carry two primary weapons, like, having an additional secondary weapon, then go to your “Custom Loadout” > “Perks”
In the “Perk Select” go-to “Overkill” and enable/equip it. This allows you to have two primary guns. This will be available once you reach Level 8 I guess.
You can go to your “Secondary Weapon Select” and keep your desired weapon. I selected a Sniper Rifle but you can pick your desired ones, some even prefer a shotgun.
This will allow you to improve your game style and win matches. Do let us know if you have any queries.