Fuck.
I feel like this means another "Resident Evil 5". Where you'll get a shit-for-brains AI that basically makes the game half as fun unless you're able to find someone to co-op with. At least with RE5 you could go online and find someone.
I really just wish they'd make like RE4 single player games.
Nothing has ever shown that the game has forced Co-Op. In fact, the game seems pretty reliant on making you feel as alone as possible, unless the entire game has changed since the last trailer was shown. What they most likely mean is that they have a demo of The Mercenaries 3D on there, similar to how The Mercenaries 3D had a demo of Revelations (I'm going out on a limb here and hoping they haven't decided to put a fully fledged Mercenaries minigame into it, 'cause that'd just mean there was never any point in getting The Mercenaries 3D).
We also know fuck all about the actual game at this point, so assuming we know what they are aiming for game play wise at this point would be nothing but presumptious. There is, from my understanding, a rather short demo on The Mercenaries, and that's about it. That is also a
very early demo relative to the game's release, which is still TBA last I knew. If you allow for two players to play the campaign at the same time, which is likely what they mean when they say there can be two players, then the maps will have to be built around two person exploration. If they are built around two person exploration, you better be ready for some shitty AI.