APC makes that external surge protector stipulation in a
list of exceptions. They make a warranty claim. Then include
numerous 'fine print' exceptions so that a claim need not be
honored. Chains of protectors adversely affect protection
only because those protectors add even more wire to earth
ground. No short connection to earth ground means no
effective protection from the typically destructive transient.

All being irrelevant to Agent_C's original post. Since a
power strip (with or without surge protector) does nothing
(remains inert) when voltage is well below 300 volts, then
either power strip will work just like a power switch.

Steve Urbach wrote:
Many $9.oo strips claim some surge protection :^)

Note APC protection plan states that it is void if you use external
surge protectors on either the line or load as they claim that it is
possible for them to interfere with each other resulting in poorer
protection than either one could supply.

Make sence if neither sees the full surge to initiate full clamping.