Should I report someone for playing two characters at once?

[Reinha]

Multiboxing is not against the rules assuming the multiboxer doesn't use his characters to disrupt others' gameplay. Keeping one character inactive in a dungeon on purpose is a disruption. He should find a way to play both characters at once with creative keybinds and macros instead of being a nuisance to random parties.

This applies if he automates one or all characters:

2.1 Cheating and Botting. You may not create or use any unauthorized cheats, bots, automation software, hacks, mods or any other unauthorized software or hardware designed to modify the Game and gameplay. In addition, you may not take advantage of game system bugs and exploits during gameplay.

This applies if he intentionally disrupts the gameplay of others:

3.2 Disruption. You may not in any way disrupt or interfere with the Game experience of other players, including the disruption of Square Enix's computers and servers.

Preventing people from teleporting by trading them, resetting a mob that has been pulled by someone else and refusing to move or perform one's role at all in a duty are examples of what could count as a disruption.