Capcom removed the part of the games code that allowed the splitscreen hack to work when they released the game on steam.

You need to install the GFWL version if you want splitscreen which can be found in this games library page along with the key needed to activate the game on GFWL.

http://imgur.com/EyRdBtq

To access the GFWL version...

1. From your Steam ‘LIBRARY’ list, right-click on Resident Evil 5 / Biohazard 5 and select 'Properties'
2. Select the ‘BETAS’ tab.
3. Type G4WLOriginalVersion (case-sensitive) in the 'Enter beta access code' field and then click CHECK CODE.
4. Select "original - Original GFWL version" from the pull-down menu and click CLOSE.
5. Wait for the Steam client to finish downloading the GFWL build.
6. Launch Resident Evil 5 / Biohazard 5

Then go here to get the Splitscreen hack...
http://residentevilmodding.boards.net/thread/461/split-screen