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:
If you level ranged DD, add a mage. It brings free water, food, portals, damage, AoE damage, crowd control, and increased PvP power.
If you level melee DD, add a healer. You can run dungeons easily.
If you level a mage, add another mage. Nothing is better than a mage :)
Or you can try any combination which looks fun to you.