This is incorrect. Two fans in series doubles the amount of pressure generated by the fan, while leaving the max flow rate the same. Two fans in parallel doubles the flow rate but leaves the pressure unchanged. This is why some radiators have two fans as it increases the airflow when needing to overcome something with high static pressure.
Fans in series should not be so close as it would cause turbulence, however, some gap will prevent this (or something that naturally fixes turbulent flow, like a radiator).