well it makes 120cfm (you said you have a 120cfm fans on each side of the hood). Basically you're not increasing the speed of air, just trying to keep it continuous within the hood (120cfm) and being fed out of the exhaust fan. Basically your using 1 fan as a booster fan. if it is not a cool tube, you're going to have some resistance and it will drop the cfm. if you have added ducting, it drops even more. Plus bends,etc... in the ducting.
I would say, go with one fan to exhaust the hood. most likely 300+cfm due to resistance within the hood and venting it out at a reasonable distance (also causes resistance). This will help in removing the majority of the hot pockets of air that can buildup in some hoods. Cool tubs don't have these pockets. you'll be able to get the grow room temps (not radiant) down to about the temp (2 -
of the air coming in to the grow room.
make sure your using a inline centrifugal fan (canfan, eclipse, elicent, etc..).