A lot of really good comments on this one.........
My input would be as follows: The OP stated that the process to be regulated is "FLOW". The main reason to add centrifugal pumps in series is to increase "PRESSURE". For example, multi-stage pumps are (largely) just stages in "series" for the purpose of building a pump that is more efficient in a higher pressure range.
If the pumps are located together, I would consider running them at the same speed (conceivably with same analog input to two drives OR if the horsepower is low, consider running both pumps with the same drive.) EITHER WAY, it will be VERY important to monitor the performance of both pumps, so that if either one goes off the curve, BOTH will go out on safety.
In the scenario as described, "suction pressure" would be monitored only for the sake of determining the proper performance of each pump within its engineered performance curve.
The process variable is "flow".
The PID concept will work just as well for "flow" as for "pressure". The key is to keep the pumps operating within their engineered performance range as determined by measuring "pressure", and to limit the "max Hertz" to keep them from going "off the curve". This would be a simple drive parameter setting at startup, backed up by some carefully set pressure switches at the intakes of EACH pump.
While you're developing the automatic system of safeties, you should consider adding (1) "overtemp" sensors and logic, and (2) careful setting of the "overload" feature of the VFD(s).
Happy pumping,
Stationmaster