Well that would make sense If I remember correctly though in DND when you dual wield you stop increasing in levels of the first class when you start another class
I assume you mean multi-class? In 5e, every time you gain a level, you can choose for that level to be in any class for which you qualify (they all have stat requirements). So you could totally do something like:
Level 1) Fighter 1
Level 2) Fighter 2
Level 3) Fighter 3
Level 4) Fighter 3 Ranger 1
Level 5) Fighter 3 Ranger 1 Barbarian 1
Level 6) Fighter 4 Ranger 1 Barbarian 1
Level 7) Fighter 4 Ranger 1 Barbarian 2
...
