You need to unlock the advanced job Ninja to dual wield (min level: 30).

http://ffxi.gamerescape.com/wiki/Ayame_and_Kaede

And you unlock the support job by doing a quest in either Mhuara or Selbina.

http://ffxi.gamerescape.com/wiki/Elder_Memories - Selbina
http://ffxi.gamerescape.com/wiki/The_Old_Lady - Mhuara