Dual Wielding

If you go to the equipment tab, (Next to housing) you will see Two Slots Below Hook, Right hand and Left hand weapons.

Use Right Click to use the Left Hand weapon.

The Left Hand weapon will be darker as it's in the "Shadow" side of your character.

Bows Can't be dual-wielded.

Rocket Launchers Can't be dual-wielded.

Shotguns Can't be dual-wielded.