Usually a line to make fun of Microsoft/Xbox fanboys when they get too full of themselves. "Teh dual wielding" literally refers to one of the features in Halo 2 that wasn't in the original Halo, in which you can hold two guns at the same time. Xbox and/or Halo fanboys really thought this was teriffic, and sometimes even say it's the best improvment or feature of the game. Non-Microsoft/Xbox fanboys tried to point out that the dual wielding of weapons can be found in games much earlier than Halo, but no one really listened. Now, people generally quote in fictional situations with them saying lines containing "teh dual wielding" to effectively portray how pathetically unwavering their faith is in Halo 2's "innovations".
Xbox Fanboy: LOL!! U TEH STUPID N00B GAMECUBE FANBOY FAGG!! HALO 2 PWNZ UR SHIT METROID LOL PWNED!!!!111ONE11
Me: Halo 2 isn't that great. Maybe you should really look into other games a little more seriously before you become so judgemenal.
Xbox Fanboy: But...but...teh dual wielding...*cries*