Rusty,
I think you might be overestimating how fast things will heat up
with a pump failure.  If it was me, I'd do a little testing -
start at cruise at altitude - and shut off the pump and see what
happens and how fast.  I think the temps will rise slowly enough
to give you plenty of time to turn on the backup in case of
failure of the primary.

Next thing I would do is drive my coolant alert alarms with
coolant pressure.  I believe that in case of primary failure,
the coolant pressure will start rising a good bit before the
temp [readings].

Next, if the (95C) thermal switch proved reliable and accurate,
I would have one for each pump.  That way, I could always fly
with one pump on, one on standby.  Perhaps there would be only 3
positions on the switch:  Bypass, ON (controlled), and Standby
(using controller, but through the temp switch).  An OFF
position would be my fourth choice and would involve a funky
switch.

In any event, having done the above, I would test both pumps on
the ground and fly using one as primary and the other as
backup.  I'd alternate pumps either every couple of hours during
a flight, or pump #1 as primary on odd numbered days and pump #2
on even numbered days.  I would occasionally switch from one to
the other in [steady state cruise] flight to ensure that the
controllers are maintaining the same temp.  I would make myself
a master-switch-bypass that would leave the primary pump running
for 10 mins or so after engine shutdown so as to prevent
internal boiling that forces coolant into the overflow bottle
and having to be sucked back into the system later.

I think EWPs are going to be HUGE.  And sooner rather than
later.  But I think failures are not going to cause problems
nearly as fast as folks are speculating now.  When's the last
time you had a water pump belt failure on your car?  It ran for
one hell of a lot longer than six seconds.

That's what I think .... Jim S.