A second Blacksite: Area 51 is apparently due next month and may well include co-op according to the game's producer Mike Jones. Mike also admitted the previous demo (which you can still download from Marketplace) was rather short but the next one will be much longer. "Next month, there will be a full length demo," he said. "The previous demo was a short one, it was just a teaser. The demo next month may have co-op and there will be multiplayer components to the demo."
A demo with maybe co-op & multiplayer, now we're talking! We'll keep you posted if any more evidence of this comes to light in the meantime.