Gamesradar preview reveals drop-in co-operative play feature

From the preview :

Or should we say maternal? It all depends on whether you’re Player Two or not. Joining Chris’ fight is a new female character. Now that Chris has a partner he needs to keep in constant touch. He has both a Bluetooth headset (and phone) and a walkie-talkie; one is for chatting with the female mercenary and the other for speaking to the outside BSAA contact, Metal Gear Solid codec style. Currently nameless, the mercenary’s inclusion means that, for the first time, you’ll be able to tackle a true Resident Evil game co-operatively online. Like Gears, the entirety of Resident Evil 5 features jump in/jump out gameplay with the AI taking over the reigns of our mysterious female mercenary when you’re playing alone.

The game is rumoured to release sometime before April next year.