Not sure what ya got there. Sounds like two of the fans maybe being used for exhaust, both adding up to 700cfm for a 640 cubic foot room clearing in under five minutes.... not bad. Props. Wish I could have made them work, they're not very noisy at all compared to my squirrelcage I wound up...
Yeah I know the exact fan you speak of, in fact I have one sitting in a pile of parts I dont use anymore.... because they perform poorly. They are in fact designed to aid your existing air handler, and with a non restrictive intake they may indeed work. If however there is even the slightest...
The inline fans from Home depot are not the greatest so maybe one coming in and going out, if you get a fan with a lil more ass on it, a well placed passive intake should suffice.