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Three things IME/IMO:
#1 Stick to one single fan. If you want more airflow, get a bigger one. That is, if you want to be able to keep the fan on while observing/imaging (one good reason to do so even when ambient temperature has settled is that it can help with dew). Once you have more than one fan they will run at slightly different speeds and will cause "beats": amplified low frequency vibrations due to slight mismatch in how fast they spin. Your scope tube/mount can easily resonate at those low frequencies as well.
#2 Install some basic speed control for the fan; 3 speeds and off is plenty good enough: Run it flat out for cooling off, at medium speed for maintaining temperature, at low speed or off for observing/imaging.
#3 Baffle your fan, i.e., mount it in a solid baffle: make sure it directs air from the back of the mirror up the tube with minimal leakage.
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