Caesar Romano wrote:

If two surge protectors are connected in series, is the amount of
surge protection available at the down-stream protector approximately
equal to the sum of the two individual protections??

By 'surge protector', do you mean something like plug strips with
MOV peak voltage limiting? If so, the upstream device will limit
an overvoltage transient and the downstream device sees normal
waveform and provides added protection only if the upstream
device fails. Actually, the varistors don't provide an absolute
clamp at their trigger voltage, but the downstream guy will do
very little protecting.

If you're referring to connecting two MOV's in series across the
line, then the overvoltage clamping will begin when line voltage
reaches the sum of the individual MOV clamp voltages. The same
transient current will flow through each MOV, and each will
dissipate part of the transient (as heat). If they have the same
clamp voltage, then each MOV absorbs half of the energy.