In my experience, the setup works quite well when designed as described above, with check valves at the output of each pump.
This setup has already paid off for me- the primary pump was originally a Varios8, which failed after about a month. The backup came on seamlessly, and kept the system alive for the three days I needed to wait for the replacement part.
This is my H Valve set up.
Currently have just one tank but I can run two tanks and one sump.
If either pump fails then A flip of a valve and will be back in operation.