Assuming the fans are both orientated in identical directions, eg --> and --> then the overall flow will be very slightly improved. I disagree with the comment above that it is actually reducing it: you are creating suction on one end not in fact restricting flow in any way, it is actually enhancing flow through the mid point of the tube.
You can see the same effect with tower CPU coolers that are designed to accept fans mounted on either side. In summary:
▓ = heatsink, ▌= fan
--> direction of airflow in below diagrams
▐▓ gives good performance
▐▓▌gives very slightly better performance than above
▓▌(suction-only through heatsink) gives poor performance
The benefit of using the configuration shown in the middle is so small, that usually its not even worth the extra money for a second fan. In all the overclocked rigs I've setup, it usually offers no more than 1-2 degrees C lower temps when under load. I should add though, that the dual-fan configuration is beneficial in case a fan fails: if the single fan dies while you are not paying attention then your miner will possibly get really hot (and maybe burn out, depending on what it is).
Remove the housing, cuz the hot air is lighter than cold air. And hot air goes up
Yes, that will help you pass junior grade physics, but in practical small scale applications like this, air density effects make very little difference when it is being forced by fans.