In a closed loop two different pumps would be a disaster.
FromThe test in series performs very well. It is as simple as adding the two pump curves in the Y or pressure direction. The little bit of tubing between the two pumps is not a measurable restriction and the curve results are very much as expected morphing the two pump curves into one stacked curve. While this does double the pressure of two like pumps, it should be noted that it does not double your flow rate. Because the restriction pressure drop curves are not linear, you generally net around 30% more flow rate from running two pumps in series. This would be the recommended method of running multiple pumps in one loop and it does NOT have to be the same pump. A modified DDC-1 and PMP-450(D5 Vario) work together very happily together.