When taking on Mental’s hordes in Serious Sam 4, you’re going to need all the firepower you can muster. That’s why you’re going to want to dual-wield weapons.
At the start of the game, Sam can’t dual-wield anything in Serious Sam 4, unfortunately. Only by finding mysterious purple orbs and gaining S.A.M. Skill points from them can he unlock numerous dual-wielding abilities that make the game much more fun to play. One S.A.M. Skill Point is pretty much given to you early on in the game. The others you need to find in brown crates.
Once you have some S.A.M. Skill Points, open up the menu and access the S.A.M. talent tree. You’ll see that if you follow down the left hand side of the tree, there are various dual-wielding skills to unlock. Overall, you’ll need five S.A.M. Skill Points to unlock them all. First you’ll be able to dual-wield pistols, then any two-handed weapons. After that, you’ll be able to dual-wield heavy weapons. And finally, you’ll be able to mix them up.
With a weapon equipped, you can switch back and forth between dual-wielding by pressing the left-alt-key if playing with a keyboard, and the right shoulder button if using a controller. If you have the final dual-wield skill unlocked, use the weapon wheel, accessible with Q when using a keyboard and the left shoulder button when using a controller, to customise the two weapons you want to use.