Dual leveling

Leveling two (or more) characters together in Classic WoW is faster and more effective. It is valid for BFA as well. All you need is a mate(s) who is always online when you are online. Or, you can use Levelbuddy.

Multiboxing leveling is like playing in an arena - some classes have great synergy. It is especially true for slow-leveling classes like a warrior.

The rules of thumb are: Or you can try any combination which looks fun to you.

Also see this famous video.