Irrelevant when it comes to venting hoods. The largest hood on the market (Magnum XXXL) is less than 6 cubic feet, x2=12CF, a 6" Vortex is rated at 449 cubic feet per minute. Assuming ducting/hood resistance and we're down to 300CFM. 300/12=25, meaning the air in two hoods are being exchanged every 2.4 seconds. A 6" Vortex is plenty to vent two hoods running 1000's, I ran that exact setup WITH a carbon filter and both hoods were cool to the touch regardless of ambient temps and everything else you're attempting to act intelligent on, nice try though.Unlike the short response given above, you really need to give us all MORE information in order to give you an intelligent answer.
ALOT of factors determine what size fan (or fans) you will need. For instance-
-Will you be using passive air intakes per light? It sounds like you want to chain them. If that is the case the exhausted heat from the first can def increase the heat load on the second one inline.
-How about the incoming temps of the intakes? What is the ambient temperature of that source you will use for the intakes?
-Will you be drawing air from outside the growroom to vent the lights? Or will you be drawing grow room air straight thru the lights and then out?
-How are the temps in your room now? (before the air cooled hoods)
See how many questions can be asked? And that is for starters. Alot of people grow mediocre bud because of piss poor planning. Properly plan your work and then properly work your plan.
Unless you are a psychic (which I highly doubt) none of us (you included) were first informed of the INTAKE paths to the hoods. My brother has done HVAC all his life so that's where my so-called intelligent decisions come from.
At least the poster was smart enough to clarify his particular setup later , which makes it better for everyone here. Details, details, details. 
