It's a difficult question to give a very specific answer to:The fan operates on a principle of creating a pressure differential between one side and the other. That differential will cause air to flow. 1 fan will take equal pressure on both sides and create a difference with a given amount of flow based on the density of the air, the speed of the fins, whether there is active control on the motor or not, etc...lots of variables. If there was no air resistance, or friction in the bearings, the fan would spin at an alarming speed as all energy input would go to pushing the blades faster. (keep that in mind)Now imagine you put a fan into a moving air-stream. (doesn't matter where that stream is from, its just moving). The fan will experience less force against the motor, so it will spin faster (if the mechanics/electronics allow) such that the energy input is put into the air. If you have 2 fans end to end, you get this result. That SHOULD result in another boost to the pressure differential, and more airflow. Whether that works in a brushless fan being controlled by some electronics, I can't say. As soon as they spin faster (from the reduced drag) they may be set to slow down to a given rotation speed. I can't answer that.I can tell you, multiple blades in a row in a ducted fan mechanically forced to spin a given speed regardless WILL boost pressure. This is the principle used by jet engines in their compressor stages to add/extract as much energy as possible to the air, getting it up to incredible pressures before the combustion stage. Subsequent sets of fans (turbines) similarly extract some of that energy to power the compressor fans. The rest converts to thrust.Another thought; It's very difficult to translate into 'suck' force because a vacuum spreads in all directions, where a push/blow force is usually more directional because air has mass and momentum when leaving a blower. If both fans are ducted end to end such that one feeds the other you'll get a boost, almost guaranteed. Because of losses and how the system actually works, I imagine the boost will be far less than double (1 fan plus 1 fan <> 2 fans).