How is it possible?

[WingsofWar]

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You can level both PLD and WAR and have them share gear and obtain their best weapons or equal ilvl, making it possible to "Main" both tank classes. Same with SCH/WHM, SMN/BLM. For most people a main class would mean that they are most comfortable with the rotation and dynamics of that particular class which usually is just one class. But its not impossible to actively be good at multiple classes, especially if they are of the same specific class genres. Just like some people are right handed, some are left, and some are good at using both hands equally.

For me i have only 1 main (PLD) as im MOST comfortable with that class in terms of rotations, movements, skills. Id be to much for me to actively be equally good at another class, so they are alternates.