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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.
What you are missing is that the fan blades of the second fan are actually causing a huge restriction in that configuration.
Your example of a push/pull configuration in a box/tower is creating more internal turbulence than a single fan, and you are probably getting more complete air movement *within* the box, but if the box is otherwise sealed, you certainly aren't moving any more air in or out of the box due to having a second fan placed in the exhaust port. In fact you are moving much less out of it due to the fan being there and causing a restriction.
If you want to increase CFM with multiple fans, then you would have to provide an individual matched intake and exhaust port for *each* fan. Always have matched ports for each fan or expect to pay significant penalties in the form of CFM.