How could the receiver possibly "know" in this situation? It just send out a signal via the speaker terminals.
If you just wire the two speakers up directly, they would be hooked up in "parallel" and would present a combined impedance load to the receiver. For example, if you had two pairs of 8-ohm speakers hooked up, the receiver would be presented with a 4-ohm load. Probably not an issue but could be a problem at higher volumes.
If you added a speaker switch to control the A/B function, there are switches that provide an impedance matching function to avoid this issue. If you google or search on Amazon for this term you will find many options.
All that said.... by the time you factor in the cost and hassle of implementing this solution, you would be better served by simply swapping receivers. I bet you could sell your 591 and find a different Denon with 7 channels that can power Zone 2 for less than a 50 dollar incremental expense -- basically the same as what you would be spending on the speaker selector.
For example, don't know where in Texas you are, but I was able to quickly find the following options in Dallas:
1. Denon AVR-791 for $140: http://dallas.craigslist.org/dal/ele/3745738674.html
2. Denon 1910 for $220 (I would negotiate this price down): http://dallas.craigslist.org/ndf/ele/3740165741.html
3. Another AVR-791 for $195 OBO: http://dallas.craigslist.org/ftw/ele/3710981838.html
4. An AVR-1912 for $250: http://dallas.craigslist.org/dal/ele/3764780767.html
1. Denon 2309ci for $135: http://houston.craigslist.org/ele/3759564734.html
2. Denon 1611 (the same as your 591 but with 7ch and Zone 2), the guy is asking $300 but that is ridiculous, you can point out that Accessories4Less sells the refurb 1612 for $200: http://houston.craigslist.org/ele/3757483973.html