One other thing that may be worth noting, I notice when I run one fan, the other fan has a reverse draft going through it ( I can see the fan blade wanting to turn in the opposite direction ever so lightly). My theory on this is that it allows yet more heat to be sent through the live fan because of not only pulling air up the back duct-way for the bottom in the closed-loop cycle, but also pulling air from just a few inches away from the other fan opening where the heating element is making half the heat. I see it as a benefit to some degree.
And oh yeah, when you get it built, take you a small clean shop vacuum if you have one and go to your pens and collect some "down" by holding the vacuum hose up high enough to only suck up the light, fluffy stuff.........and go put them in your bator with your fan(s) on and door shut. Cut your lights on, and you can somewhat do your own "air-tunnel" test to see the effects of air-flow with one fan / two fans situations.