The figure below depicts an amp running unattenuated, at full power, into a pair of speakers. In this case each speaker is driven by half of the amplifier's full power.
The figure shows only two speakers, with the amp running at full power. But the key conclusion - that each of the two speakers sees half of the power generated by the amplifier - also holds for the case of the amp running at less than full power. It also holds for the much more general cases of pairs of identically rated speakers networks or more general amplifier loads.
