You're causing cavalvation(sp) int. wind reduction or turbulence which reduces overall effect. The 2-fans would work better if placed side-by-side mounting. If you have the original mounting setup, note the model# and find what its specs are. I gather it would be 1/2 CFMx2 for both fans. By heck, if it working for you, go for it.

tada -----Willy Happy