I just bought the 42LM3400 & had the same problem with dual play on Gears of war 3, I found it worked better if I turned off the 3D support on the game itself then tried the dual play on the TV & it worked but you don't get the 3D but there is a little ghosting not to mention that player A will get the top end of video scenes while player B will see the bottom end. I was disappointed with the dual play all in all but the technology isn't perfected yet as it would seem.
I have an LG 32LM3400 with passive 3D feature including Dual Play . The problem is that I can not get to activate the Dual Play option. Through the remote control 3D button and when playing in split-screen co-op games. It just give me the 2D to 3D conversion option. But when the TV is connected to a TV channel, Dual Play can be activated. I tried to connect my Xbox with two different HDMI 1.4 cables but I'm still getting the same problem. Do you think that I should upgrade the TV firmware(knowing that the TV was released in June 2012) or is there any special configuration to make such option work ?
PS: the games I tried were Gears of War 3 and MW3