On some screens or in some trailers you might have seen that the Green Beret can dual wield machine guns. This is also featured in the demo it just took a small bit of trying to find out!

Firstly it isn't possible in the demo to dual wield the machine gun you start with (there doesn't seem to be another one), the germans use their trusty mp40 so you'll need to pick those up. To do this first kill a soldier with a mp40 then run to him and press F to pick it up. You'll loose one of your other guns though. I'm sure there's a way to decide which one but by lack of a manual I don't know. Ok so now you have one mp40 but you'll need another one so when you see another dead soldier with a mp40 walk over it and you'll pick it up.

After you've done that you'll have two mp40's in your inventory you'll just need to activate them. You'll need to use the é key (might be different on your keyboard it is under controls, other, next item) to switch from you're grenade to the icon you see below. (don't mind the bad performance I was concentrating on dual wielding guns rather than playing well) After you see the icon that is marked with red on the screen press the left ALT (also default) to activate it.