Alright well we all know that you have always been able to split-screen in all of the Gears of War games with Gears of War 3 not being an acception. But now we can split-screen in the Gears of War 3 Beta with a guest. This is a bug in the beta, but I see no harm done as it will be a feature in the full retail version. Here is how you do it.
1: Start up any game that you can play with a guest online like Halo Reach, Call of Duty: Black Ops, etc.
2: Sign in a second controller as a Guest in that game.
3: Navigate to quick launch and start up the Gears of war 3 BETA.
4: Press start on the title screen with your gold account.
5: A pop-up will ask you to turn off the second controller. DO NOT DO THIS.
6: Next you’ll need to find a Friend or someone on your Recent Players list who is on the Gears of war 3 BETA and join their session.
7: At this point two things can happen. You will join the session along with your guest or it will not let you join but instead put you in a lobby with your guest.
8: If you are in a lobby with your guest you can then safely search for a game with your guest.
Note: A minor bug may occur if you or your guest press the start button for any reason during a game, the opposite player will still be able to continue playing, but the screen will turn black and white as well as becoming all blurry. To fix this you must both pause your screens at the same time and un-pause at the same time. Problem Solved. Have fun!