the conventional way to cool computer components is to move air over and/or through them. In ideal circumstances a pair of drives will have an airgap between them - the cases shouldn't be touching, or so close to one another that air can't circulate freely.
Once that's sorted out, the priority is to move air through the case - normally from front to back, and exhaust it to the room in which the computer is situated. The greater the volume of air that moves across the drive cases, the greater the cooling effect will be, so a big fan is helpful. The important thing - and one that a lot of people overlook - is that it's no earthly good having a huge fan sucking air in at the front of a case if there is nowhere for the warmed air to go after it's done its job. Thing about intake and exhaust as being parts of the same equation - one won't work properly without the other. You can have one fan doing both jobs of course, and that's what happens in a normal Power Supply Unit - the fan pulls air into the case via perforations or vents in the sides and front, and exhausts it through a grille at the back. This isn't enough for things like CPUs, so they normally have their own small fan mounted on top of a heatsink. This pulls air through the fins of the heatsink, and disperses it into the case - along with its warmth. That's why it's so important to get warm air from inside the case out into the atmosphere as rapidly as possible.