Bigben2k stated this earlier, but it seems to have been lost somewhere along the line. If you put those two pumps in series and either one dies, the flow will plummet. These pumps work by spinning an impeller inside a casing. The casing takes input at the center of the impeller and sends it out at the periphery. If impeller no spinny, flow no go (very well).
There's nothing wrong with running them in parallel, though I'd strongly recommend a check valve at each pump's outlet. Again, if one pump died it would be a "short-circuit" and you'd have some flow going where you wanted with the rest flowing backwards through the first pump and into the second pump's suction. If the resistance of the stopped pump was comparable to the circuit resistance, total flow would go up, but flow through each circuit would be less than if the dead pump wasn't present.
In parallel, total flow will be higher than either pump would do on its own provided the discharge pressure of the combined pumps was below the dead-head pressure of both. In series, the total flow would also be higher than either pump individually so long as the zero-head flow of both pumps was higher than the resulting total flow If one pump had a zero-head discharge lower than what the other pump would do under load, the "weak" pump would be nothing more than another (albeit small) resistance to the "strong" pump.
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