You can put two pumps in series to double the total head pumped. But beware: Using series or stacked pumps to achieve the head required in some drainback systems can cause big problems. If one pump quits during colder months (and they all quit eventually), and the other keeps pumping, it could lift the water just high enough to where it can sit and freeze. The frozen pipe can burst, and then the system could pump all the water in its drainback tank into the attic. This is one reason for the less-than-stellar reputation of drainback systems in some parts of the United States. The solutions to this head problem are to raise the drainback tank to a level that will accommodate the head of the chosen pump (see how James Dontje solved this in his article in HP120) or to select a higher-head pump if available.
Many new pump installations will need a flange set or union set to connect the pump to the piping system. Make sure you have the additional parts you need before you start work on the system.
Installing two pumps in a parallel piping arrangement will increase the flow of the circulating fluid, but will not increase the total head.
All the SHW pumps mentioned in this article are classified as “fractional horsepower” and don’t require a separate electrical circuit. However, fractional hp pumps do require a disconnect—an appropriately rated switch or breaker or a UL-approved cord and plug connection.