Good point when using them as a vacuum pump, but all are also compressors on the discharge side when their inlets are full open.
The rotary vane and dry scroll pump designs are used as superchargers, which put out high flow at an atmosphere or less boost.
To double the output of the secondary compressor, the first pump would have to maintain 15 psi pressure on the second pumps inlet. That means it puts out twice the rated output of the second pump while maintaining 15 psi.
As an aside, the way to throttle a blower for least horsepower usage is on the inlet side, so that the impeller is operating in a partial vacuum versus in compressed air when throttling/deadheading the discharge side.