How is it possible?

[Gormogon]

To me, a "main" is what you play most and what you are best at.

A "main" is mainly used so that people don't get greedy with gear

Regardless if it's your best job or not your main and the groups main are priorities over secondaries.

You can't have two mains. Because you can't play two jobs equally or have two best. There's always one you play better than the other.

According to who? People are more diverse than that.

If a player is dedicated enough that player can become a jack of all trades if that is what the player wanted to do.