In my CM 690, I actually tried one in and one out in the top before I went WC on the rig. Only way I could get it to work was to insert a divider that made the fwd fan air go down to the bottom of that divider and that screwed my air flow up everywhere else. The other thing I tried was to make the fwd fan pull from the 5.25" bays, cooling my DVD rom drive which I never use anyway. LOL. With a Black Ice 240 installed up there, you need both fan slots. Without that being installed, I agree that the fwd fan slot is pretty useless.