Four idential speakers, two each side, will only screw up the stereo imaging. At best, you only increase the volume of out put 3db. I wouldn't bother.
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If you put two speakers, "stacked" one on top of the other, you have made yourself sort of a line array, which has some advantages, particularly with regard to imaging. Put the top speaker upsidedown so the tweeters are adjacent. At one time "stacked Advents" was a highly regarded and relatively inexpensive speaker system. (Roughly $600 for four, if I remember correctly). Apart from line array imaging advantages, the objective was not 6dB more loudness, but rather half the power into each speaker.
If you put the speakers side by side, you might have a problem, but if they are good speakers each side by side pair will image as one speaker between them, so things might be OK. This is the sort of thing that you need to try out, as theory doesn't always give the best answer.
I should have been a little clearer...each speaker will be powered by it's own amplifiers.
They are Infinity Kappa 9 speakers. Each one would be bi-amped with a Carver Silver 9t monoblock on the bottom and a pair of Sunfire Signature stereo amps running the four tops. Because of the speaker design, it would be impossible to stack these...so, they would have to be side by side. The bi-amping would solve the amp strain question.