This tutorial need an improve version https://learn.unity.com/tutorial/fps-mod-dual-weapon-wielding
This dual-weapon wielding have so many bug but I found a way to solve them.
Firstly, you need to choose which side you want to use for your single weapon. Open player from the Prefabs and complete the down "Player Weapon Manager" component like you saw from the tutorial with the side you want to side. Complete the up "Player Weapon Manager" component with the another side. You need to test your weapon with a blaster in a first hand and a shotgun in an another hand to check if it use the ammos of the side you want.
For the dual weapon, you need to create two similar weapon, add them in their names right and left in the appropriate side. Change their aim offset y=0 and x= to the x of the DefaultWeaponPosition. Double click to the projectile you use and set the "Trajectory Correction" to -5.
For the single weapon, you need a fake gun which you set the max ammo to 0, you disable the "Weapon Fuel Cell Handle" and "Overheat Behavior" component. Set everything in the scale to 0.01. You will have a gun and an invisible gun that will feel like to have a single gun.
Watch out, you can't have two same weapons in the inventory. If you want to do have it, you need to duplicate the weapon with an another name even if you use a fake gun.
Sorry if I can't explain clearly but I willl answer some question if you are stuck.