Why can't I use Dual Wield? ⚔ in Mini Militia Doodle Army New Version

On the new game version, Dual wield is only available when you reach that weapon required level. Dual wield isn't related to your account level. 
As you can see on the screenshot below, different weapon levels are required to get the Dual wield. 

Here, you can check that on the Magun gun, since it's a level 10 the Dual wield is unlocked.