During a Q&A with the creative minds behind Resident Evil 6, a couple of new tidbits of information were announced. The first interesting bit of information was the addition of crossover sequences into the game. At certain points in the game there will be times when 4 characters will meet up and have to work together. During these times players will have the chance to initiate crossover and play the sequence in 4 player coop. While it is possible for you to play these sequences with friends, it is unlikely seeing that your friends would have to be in the exact same area of the game that you are.
This could be an interesting feature if it works right. I guess we will have to see how it turns out when the game releases on October 2 for the Playstation 3 and Xbox 360.