http://www.pumped101.com/#series_parallel
http://www.pumpworld.com/parallel_pumping_identical.htm
http://www.engineersedge.com/fluid_f...mps_series.htm
parallel to increse flow
series to increase pressure
then there is always failover
of course this assumes the rest of the loop can deal with and benefit from a 2nd pump
pump-heat (kinetic energy) is real, and you quickly see diminishing returns