Your fan pulls air through it when the impeller is spinning, when the impeller is not spinning in one fan it creates a an obstruction making it harder to push or pull air through with the other fan.
I am guessing you don't want to be having one fan running constantly in case it causes failure due to over use running at full pelt etc.
I use a thermostatic twin speed fan which runs on the lower setting and boosts its extraction speed if the temp goes above my set ideal temp for the most part my fan is on the low setting which doesn't strain the fan from constant high speed use. I have my fan running 24/7.
Alternatively you can use and over sized fan and use a fan speed controller to reduce the speed and strain on the fan.
Most fans have a thermal cut out if your fan should fail, I keep a spare fan just in case this ever happens.