Your amp will be seeing a different impedance driving both so you should also take this into account.
Will using bothA and B spkr out half amps power?
I have an NAD int amp powering Usher speakers. Im experiementing with a old Paradign sub which is passive so I have it hooked up to the B speaker outputs of the amp. When I am running both A and B, does it mean I am running less power to my Ushers? Seems to me it would be half power. BTW-the sound quality without the sub in the system is better, but Im curious about how the power is divided when A and B are engaged. Thanks.