Can I increase air volume by adding more fans?
Its been a long time since I did any fluids, and even then, it was not much. But I'll take a shot just from simple logic.

Suppose we have a single fan at the front end of the duct. It is rated for 650 CFM, and without any duct there, you will get essentially that. But suppose you put this fan in a duct. Moving air will see friction against the walls of the duct, slowing down the air flow. The back pressure reduces the flow, so you really don't get a full 650 CFM out the end of the duct. Of course, bends in the duct hurt even more, but you said it was a straight duct.

Now, suppose you add a second fan at the tail end of the duct. This is sucking air out at a nominal 650 CFM rate too. But in the end, all it does is give you something closer to the full 650 rating for the two fans. You don't get 1300 CFM, you get roughly 650 CFM.

With two fans in parallel, both feeding into the duck (or both pulling air out) if they are not limited by air flow through a duct that is big enough to handle one fan, then you might get more air through in theory. But the problem is, now you have higher velocity air. The link that gregmac provided tells us that pressure losses are proportional to the square of velocity. If you could double the velocity (and in order to double the air flow, you must double the velocity of the air as it moves through the duct) then you would quadruple those frictional losses. 

It gets worse. As is pointed out in gregmac's link, with two fans right next to each other, the air flow into those fans will interfere with each other. So you really wont achieve the full rating for those fans if they are set side by side.

How about the 800 CFM fans? If they are too big for the size duct you have, then you again will be limited by the back pressure. You may end up with only a net of 650 CFM anyway.

It seems to make most sense to have a second duct. Run two ducts in parallel, both adequately sized for the fans they are fed by. Yes, I know this may not be an option.