In most cases, the A + B terminals of most recievers are wired in parallel. Two 8 ohms speakers in parallel present a 4 ohm load to the amplifer. Most amps can drive a 4 ohm load at generaly does cause problems.
One of the mark of higher quality amps is that they can drive low impedance loads, 3.5...perhaps even 2 ohms. They need to be able to both provide the additional current necesary, as well as not exceed the ratings of any of their internal componants.
If they exceed their current driving ability, clipping occurs. If componants exceed their power ratings...POOF.
If you keep the volume relatively low, the amplifer should be able to provide the additional current, without breaking anything. Just don't push it.
-Chris