Very bad idea. First of all you will only get an additional 3 dB out of it. Worse, you will permanently damage whatever imaging you have. You can split up woofers that way because the wavelengths they operate at are very long but, you can not do that with midranges and tweeters. At shorter wavelengths the speakers start speaking in two voices instead of one. Tweeters have to be no farther than 1/2 inch apart to speak in one voice up to 20 kHz. When you move them apart like you plan on doing you are listening to a choir instead of a single voice and you smear the image and detail.
Imaging is far better with four speakers than two. If your speakers are using identical technology (martin logan uses ribbon tweeters and both pair of speakers I use are of same impedence and sensitivity - 4 ohm and 92 db) then you are simply adding more speakers without disturbing the manufacturers musical aim/result. It is akin to setting up a for a concert with the need for more speakers and more power to push them because you have the space that will allow for it. You should be cautious about placing two differing mfg amplifiers in the same system. I researched and found that the NAD 275BEE and the Parasound 2125 vs are pretty close to being identical in nature, especially in output. The Parasound is great for the 35XTi bookshelfs because it comes with a filter that I use to negate any frequency of 40Hz or lower from getting to the bookshelfs as the 35XTi's only go down to 50Hz. All of my speakers are bi-wired with AudioQuest 5 wire.
So, yes, mixing various brands of speakers together with different amps and wires and whatnot, will likely degrade the sound. But if you pay attention to specification detail and only mix like sounding manufacturers, the result will not have any of the negative issues others have brought up in this thread.