[Skulemate]

If you're interested in reliability my opinion is that the pumps should be arranged in parallel, regardless of whether or not this is optimum. That's because it's easier to set up check valves to keep the flow going in the right direction if one pump dies than it is to set up a bypass, which would be needed if the pumps are in series (I doubt that you'd get a whole lot of flow through a dead pump, as myv65 and others have stated).

And one more thing. Myv65's statements about pumps in series versus parallel are dead on... couldn't have said it better myself.