Valve teased a crossover between Resident Evil 6 and Left 4 Dead 2 last week with the following images.

L4D2 and RE6 crossover screen 1

L4D2 and RE6 crossover screen 2
Left 4 Dead 2 characters Nick, Ellis, Coach and Rochelle will be available as playable characters in The Mercenaries: No Mercy. I have never personally enjoyed The Mercenaries as a game type, but I also have not played much of Resident Evil 6. The short video that was recently released looks great. Capcom has an amazing engine that they work with. The inclusion of characters and special infected from the Left 4 Dead franchise is a natural fit and a smart move.
While the console version of the game appears to be getting the short end of the stick, the PC port has taken a long time and the little bonus is nice. Resident Evil character skins are also available in the Left 4 Dead 2 Steam workshop.