You can place them however you want. If you want to have negative air pressure, leave them both in the back. If you prefer positive air pressure, place 'em both in the front. Balanced? One front intake, one rear exhaust.
Quickly:
- Positive pressure: Front intake fans bring in fresh air and force dead air out the back
- Negative pressure: Rear exhaust fans force hot air out, thereby drawing in cool fresh air from front to stabilize pressure
- Balanced: Intake fan brings fresh air in, exhaust pushes it out
I used to be a proponent of balanced air pressure, but now I'm leaning more towards negative air pressure as a better (and quieter) way of cooling.
Best thing is to just play around with their positioning and see which best suits your needs.
