Is it possible we were a little premature in abandoning the notion of a simple check valve to prevent back flow on parallel connected pumps?  It would solve ALL the problems at the cost of reliability of the check valve.  Here is my reasoning:
A dirt simple check valve (it doesn't have to be air tight - it can leak like a sieve and still work splendid - it just has to BE there - will be maybe 100 times more reliable than the pumps.  No motors to fail and one moving part that hardly ever moves is going to be more reliable on a really BAD day.  Couple that with the fact that the check valve is only relevant during single pump operation.  Both pumps operate only about 5% of the time, so 95% of check valve failures have to occur during single pump operation and the failure will remediate immediately by turning on the other pump.  Further, if the check valve fails during dual pump operation, it will not be known until one pump is shut down, and you can go back to dual pump operation immediately and neutralize the failure.

You are already going to automate the switching on of the idle pump in case of over temp condition, and the same circuitry will work with parallel pumps, with the added advantage that instead of 85% and 130% performance respectively in single and dual modes, you will have 100% and 180% capacity.

Mightn't it be useful to revisit parallel operation now that we have some series figures? ... Jim S.

Russell Duffy wrote:

single pump running in the series configuration flow was only ~15% reduced from previous flows without the spare pump. However use of both pumps increased flow by ~30% over a single pump big question is still the flow rate with one pump seized.  When I get my pumps, I'll try a simple test with a garden hose so see if I can figure out how much

Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T