Is the multiple signal method over-testing the EUT?
The short answer is yes; however, you have the option to reduce to a single signal, or tone, if the EUT fails. Remember that the unit is being exposed to multiple tones and each tone has a field strength equal to the required test level. Each of these tones will have slightly different phases, so there will be additive and subtractive components to the total energy presented at the EUT. The peak of this total power is only reached when all the signals are in phase for a relatively short occurrence.