The presence channels on your receiver are so that the mains can play only what is supposed to be heard through the front speakers when the audio is decoded directly while the DSP effects are outputted through the two additional "presence" speakers. In my opinion, that is a good thing. Secondly, if you wired them in parallel, the speaker with the lower resistance (impedence) would be the one that plays louder. V=IR --> I = V/R. Voltage is constant, but resistance varies so current (I) goes up when resistance is lowered. Simple Ohm's law application.
