According to Rely on Horror (via 4gamer and Biohazard France), Resident Evil Revelations 2 will not feature online co-op for the game's story component.

According to those sources, Revelations 2 will feature local co-op only. If you were planning on playing through the story with a friend online, you had better think again. Single player will also feature a bit of "co-op" in that you will be able to switch between the two characters on the fly.

Where does the online component come into play then? Well, it will show in the game's "Raid Mode." There's just one problem with that. Raid Mode will not feature online co-op at launch. Online co-op for Raid Mode will be added after the final episode is released in March. This is according to the latest Raid Mode news from Capcom Unity.
Will Raid Mode feature online and/or offline co-op?
Initially Raid Mode will be offline only (solo OR split-screen co-op). However, online play will be added shortly after the final episode (and retail disc) launch in March. The team has completed work on the story mode content and is now ensuring online Raid co-op will function as expected; while this means a slight delay in online play, the feature will be present down the road.

Raid Mode is essentially a wave based game type. The initial Episode will include 54 Raid Mode missions, Episode 2 and 3 will add 36 more missions, Episode 4 will add another 68 missions. You can learn a bit more about Raid Mode in the latest video that Capcom posted.