adding second set of speakers

Yeah okay I did silly stuff like this when I was a kid too. Stack em, more louder, cool. 

But I can guarantee that is all you will get more of, volume.

One trick you might try, that might actually be not just louder but better: Pick whichever pair you like best, set them up as best you can for imaging. The other pair, open up and disconnect the tweeter. Then place these down on the floor facing a wall in a corner. Since these are 2-way speakers even with the tweeter out the woofers go quite high so use a pillow or blanket to absorb as much of that as you can. You are basically, in other words, using them as subwoofers.

If you can set them up so you can't hear anything from them but low bass then you should be able to find locations that get you quite a bit more extension and improved imaging at the same time.  

well now, that is interesting and i would have never thought of doing the disconnect the tweeter thing.... I do have two excellent corners on either side of my listening position...

i fully agree with millercarbon here--adding more speakers, especially of a different brand, will give you more decibels, but will very likely sound worse--you're probably better off with one well-placed set