We've been getting a lot of people new to audio or otherwise unaware of how audio works and look at speakers like ordinary appliances. Some people are content to buy a car they've never driven before (I'm sure they're in the minority). Just like a car, speakers are highly subjective. Someone may think those JBLs or those Yamahas are the best thing since sliced bread and others may hate how they sound, and neither of them would be wrong.
As to combining two sets of speakers to "reduce pressure" as you put it, that's not going to work. The rudimentary of a loudspeaker is that it takes power from the amplifier and if the amplifier is unable to supply the power the speaker requires for a given output level the amplifier will going into clipping. Clipping presents itself as distortion. That distortion will sound unpleasant and you should turn it down to avoid damage to the amplifier.
Now speakers on the other hand, they're not to smart. They'll ask for more power than they can handle if you keep turning up the volume. Think of a kid who decided to have a second helping of mom's lasagna because it tasted so good and overeats to the point of causing physical pain. If your amplifier can provide more power than the speaker can handle it will cause the speaker to surpass its thermal and/or mechanical limits. Speakers can handle a surprising amount of power for a split second but too much power for too long or a huge aplomb of bass can damage the speaker.
Back to combining speakers. If you connect a set of speaker that is 6 ohms and a speaker that is 8 ohms to the same amplifier channel (this applies to A/B as well) the resulting nominal impedance is closer to 3 ohms. Since speakers are reactive loads it's possible that impedance could dip as low as 1-2 ohms. Less resistance means more current flows, and the receiver's power supply cannot handle that kind of load.
If you have questions about any of this I will try to answer if I can.
