I had an idea. What would happen if instead of a 6 inch fan, I used two 4 inch fans that have been put in-line and modified to be counter rotating (flip a blade and turn it around)? Would it boost pressure or airflow more than a single, larger fan?
No, pretty sure you can't. I remember reading this at a computer forum a few years back, so my memory is a little fuzzy. But, I think you'll burn out one of the fan motors because the first fan will be blowing higher pressure air into the second one, making the second one spin much faster it's rated to. What you're trying to do is make a turbine, so you would need to connect the rotating axis of the fans together, so they can rotate in unison. Also, I think you need a stator fan in between them.