Geoff
Could I suggest that the rather miniscule air displacement [~~8 CFM] of the blade fans you mention is also assumed as based upon CFM@FAD [free air delivery] ie., back to atmosphere, therefore the fans are displacement air acceleration elements, not air pressure force elements
So any restriction placed in the discharge plenum of the fan will reduce the FAD by a rather complex
calculation amount ...however simply put, the easiest way to understand the reduction in FAD [by restriction] in such basic elements is by an inverse increase in current consumption of the electric motor
Direct piggybacking two fans of
7 CFMFAD [on one common suction] would possibly provide ~~
8 or 9 CFMFAD total prior to the restriction
The same two
7 CFMFAD fans with independent suctions will provide ~~
14 CFMFAD prior to that same total restriction........
I hope this makes some sense........Derek