Your NAD, like most solid state amps, undoubtedly has a very low output impedance (likely a very small fraction of an ohm). As a consequence of that it will "try" to put out a voltage waveform that is proportional to the music signal going into it, regardless of what speaker or speakers it is connected to. It will do so until the combined currents that are drawn by the two speakers reach the maximum amount of current that it is capable of delivering. So the power does not get divided in half. Each speaker draws the same current (and power) from the amp that it would if the other speaker were not present, until the amp reaches the limit of what it can deliver.
Re Xti16's comment, the maximum amount of power delivered by the amp (at the clipping point, when the amp has max'd out) will undoubtedly be considerably greater than with only the Usher's connected, due to the lower combined impedance when the sub is connected. However, the maximum amount of power that can be delivered to the Ushers themselves cannot be greater than if the sub were not connected, and with an amp such as a NAD that presumably does not have large current reserves, it will probably be significantly less.
It is not easy to be more precise than that because many variables are involved, including the facts that the impedance of the Ushers and the sub are both frequency dependent, and neither impedance is purely resistive (meaning that voltages and currents get out of phase, to differing degrees at different frequencies). And of course the maximum amount of power that can be delivered by the amp varies with load impedance.
But as a rough rule of thumb, the lower the impedance of the sub relative to the impedance of the Ushers, the greater the fraction of the amplifier's maximum power capability that will become unavailable to the Ushers.
Regards,
-- Al
