In an odd turn of events, split-screen play was not officially supported for the PC version of Resident Evil 5 during the recent migration of GWFL to Steam. It was in the code, ready to be used, and a fan-mod made it possible in the prior release. But on Steam, it was hard-locked, with Capcom basically responding to fans by saying: "tough."
After much pressing Eurogamer has gotten its hands on a proven method for split-screen play, and you can find the instructions below. It's odd that Capcom wouldn't make the effort to support the feature in this clearly still-popular game, but there you have it.