There are certain points to be kept in mind and certain precautions to be taken when operating pumps in this manner.
For Series operation
Both the pumps should have the same capacity.
The pumps should run at the same speed.
The first pump should be started first and then the second pump should be started.
The casing of the second pump will have a higher pressure. The casing should be designed to withstand it.
The mechanical seal of the second pump should be able to withstand the high pressure from the outlet of the first pump.
For Parallel operation
When pumps are operated in parallel, their output increases to close to double the output of an individual pump.
The pumps must have the same flow rate