Dude, one 6" inline fan will work fine trust me. I am going to upgrade to 6", but for now my tent stays barely a couple degrees
above room temp and I'm only using a 4" Vortex on 1/2 power to cool a 400w HPS. I have no problem with loosing pressure or
suction by the time air is pulled through all of that. My filter has a high enough CFM rating to make sure I won't lose much pressure,
and it's a relatively short duct line. And I can always turn the fan up if I need too. It's on a speed control switch.
I have my filter on the end, with air being pulled through that first, then into the air cooled hood, then through the fan and
right into the ceiling. It's all in one nice setup, and I don't have to worry about dual fans or running two sets of ducting. If
I was running a 1000w light, THEN I would probably run two fans. But otherwise, one fan will work fine as long as you exhaust
outside of the room and you don't re-use the warm air.