News broke today:
This isn't a full game crossover. This is a special DLC crossover with free L4D 2 DLC going to RE 6 and free RE 6 DLC going to L4D2.
RE6 gets a special Mercenaries: No Mercy mode where the L4D2 survivors (Ellis, Nick, Coach, and Rochelle) are playable and have their own loadouts for that mode. Witches and Tanks will also appear in this mode. So if you always wondered what happens if "Leon startled the Witch!", now you can go find out. RE6 gets this free DLC come April 5th.
In L4D2, the Lepotitsa, Napad and Ogroman from RE6 will appear as additional zombies starting sometime in the Spring as free DLC for L4D2. No word exactly how these three zombies will function in L4D2, though it looks like they share similar body types as three of the Special Infected. It would be sad if this DLC was just reskins of existing Special Infected.