Hello johankj3232,
I have LG LM6610 and here is the way i operate the dual mode capability,
First of all, this option is for games that when played in co-op mode give splitscreen
e.g uncharted 3, resident evil 6
player 1 to the right, player 2 to the left. Or player 1 above and player 2 below.. etc
the dual play option will make each player when look into the screen through his glasses, see only his side of splitscreen, so he does not see the screen splitted

how to turn it on
1- enter co-op mode of your game
2- press home button from your remote
3- search for the dualplay app in the bottom of the screen
4- press on it to turn it on
( in some games, it will give you 2 screen mode options)
5- each player should wear one of the dualplay glasses ( each glass has a written letter on it, player A or player B)
wish that works ^_^