In multi player mode. I was playing a friend last night in the Archives. And it had the upper level blocked off so you couldnt go into the other side of the upstairs. But, when I played on another friends it was open and we were using three players not the two. Is this a block so two players have to move around more and not cover so much more ground hunting for each other. Or is there an unlock that you have to get to open this section?