The two Marco UPE2/e pumps in our system are both powered on all the time. There are no change over valves in the system.
They work well together. What typically happens when we open a tap and the water pressure starts to fall is both pumps come on for a second or two, then one takes the job of keeping the pressure in the system. We do not have an accumulator in the system.
None of our basin taps or shower heads flow more than 1 gallon per minute at full flow so in normal use I have never seen both pumps running together. I imagine they might run together if we ran three or four taps wide open at the same time, but that seems a rather artificial situation.
The downside of the UPE2 size is that it runs lower water pressure—down near the minimum considered acceptable for household water supply, about 23psi. We think it’s fine. The UPE3 bumps the pressure up to 36psi/2bar. I think most people would find that completely acceptable.
With this style of pump it is important to have a fairly fine screen on the intake water. This is not so much to protect the pump but it keeps a small bit of debris from jamming in the non-return valve, which is quite small. Not any more problematic than most other pumps, but it is really the only likely place to have a problem.
We have had this installation in full time use for two years, trouble free.
Annapolis, MD, USA.