This Is Not My Work. Credits Go to HideInLight for figuring this out, and ToCA EDIT for creating the xbox controller EMU.
This Mod/code gives you the ability to unlock the splitscreen code left in the game from the PS3 release. To play this you need a controller (doesn't have to be Xbox 360). I am explaining how to do this so as to help advance this little mod (If you can please make it a mutator), as well as to bring it to a wider audience.
By default you are Player0
So if you want to add more players without messing up the controls or crashing the game type these into console (tab default).
If you want to remove a player do this:
depending on which player you want to remove(never remove or add player0)
Use "SSSwapcontrollers" to switch the player controlled by player 1. this is useful for testing out the splitscreen.
Now to set up your controllers: if you have an xbox 360 controller follow this tutorial Pcsplitscreen.freehostia.com
If you do not have a 360 controller, download the Emulater from this website Tocaedit.com
go into the "xbox360cemu.ini" file in the "xboxcontrolleremu" file that you download from tocaedit, and set up the buttons as you want them to be set.
If you only have 1 controller then change where it says [PAD1] to say [PAD2] and vice versa.
copy xinput1_3.dll, xbox360cemu.ini and dinput8.dll into your game directory. start the game, debugcreateplayer 1 and you should be able to play splitscreen.
This should also work in all UT3 mods, you just have to toggle the input.cfg file to suit the button set up
Make sure your screen percentage is set to 100% or it will appear blank.
If i have left anything out (or if you have figured more out) please let me know.
Ps. There Is a similar code/hack for Left 4 Dead which can be found here Moddb.com