extremechanical,
Although there are notable exceptions as noted above, in most circumstances, pumps in parallel have the same suction and discharge conditions. Therefore if you don't closely match the performence curves, one pump will end up doing a lot more work than the other and hence you won't get the flow you want from your system. It is difficult to get it precise and if you look at the amps going into "identical" pumps in a parralel configuration, you'll always find one doing more than the others, but hopefully not more than 10 to 15%.
For series pumps, flowrate is identical, but the added head is dependant on each pump and the steepness or variablility of your system curve or discharge pressure. E.g if your normal discharge pressure into a system or vessel was 50 bar then you would size pump A to do the duty. If the pressure in your vessel or system then went up to 70 bar for some operating reason, then you could add pump B with a differential pressure of 20bar to do the duty, making sure that the suction side of the second pump and its seals can handle the high inlet pressure. So long as both your pumps can handel the same flowrate, then the curves can vary, but the design flowrate should be the same or similar.
Think of this like a wagon train - If you have two horses side by side and one is a cart horse and the other a pony, you'll find it difficult to go in straight line (parallel pumping)and one will do much more work than the other - two "identical" cart horses will work much better. If you string the pony in front of the cart-horse you will get traction from both of them (series pumping). A little odd to think of it that way, but it might help to get the picture across.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
