You can play the campaign of Resident Evil: Revelations 2 in co-op, but not online.
4gamer spoke to producers of Resident Evil: Revelations 2 and made out with a few details concerning story, setting and more.
The site confirmed that while you will be able to switch between characters in the single player, and have a co-op mode, it will only be playable offline.
The game's atmosphere wil favour traditional horror found in classic games, starting you off with no no weapons. Furthermore, Revelations 2 will not have a connection to the first game, storywise.
This follows the recent reveal that Raid mode will also be offline-only. Except in the case of Raid mode, online mode will be added to it later.