Here is the news article from EGMnow with the details:
The upcoming PC edition of Resident Evil 6 will arrive with a splash of Valve in that tall drink of Capcom, the two companies revealed today.
Resident Evil 6 x Left 4 Dead 2 will add Left 4 Dead 2 survivors Coach, Nick, Ellis, and Rochelle to the Resident Evil 6 PC-exclusive mode The Mercenaries No Mercy. Two Left 4 Dead infested, the Witch and a mini-Tank, will also be among the game’s enemies. The crossover content will be free, and available for download April 5.
Sometime this spring, Valve’s Left 4 Dead 2 will gain three of Resident Evil 6′s enemies—Lepotitsa, Napad, and Ogroman.