I attached 2 fans facing (the output of one fan is fed into the output of another) each other and sealed together with insulating tape. When one fans runs (call this power fan), cause of it's airflow, the blades of the other fan will also run (call this turbine fan); the other fan will also generate an EMF. I'll measure that. It's a measure of RPM, and so airflow of the turbine fan.
The power fan is runs at 24V (overvolted), it generated 1.42V in the turbine fan.
Another identical fan's output was attached to the input of the power fan and roughly sealed. This fan was also given 24V.
The total output of the turbine fan increased to 1.97V, that's roughly 1.387323943662 times the output of a single fan. Assume it to be less than 1.5 in perfectly sealed condition.
However notice, with 2 fan do not expect the power consumption to rise to 2 times. That's cause the second power fan is running at higher RPM cause of air flow from the 1st power fan.
So unless you're really running out of area, it's not a good idea to run fans in series.