Can't argue with experience (as above). My only comment is that the laws of thermodynamics exist - that is, the heat transfer is a function of both the volume of water
and the temperature of the water
used to transfer heat. Two units may work
well on one pump but you effectively cut the volume of water in half (actually less because of flow resistance) thereby decreasing the efficacy of heat transfer. If one pump works, two will work
better - it depends on your needs, comfort and ambient temperatures.