I used to do a lot of speaker combining (before I was married), and achieved some results that were phenomenal with really run of the mill speakers. I live across the street from a Goodwill, which up until 3 or 4 years ago was a constant source of fun, cheap vintage equipment - McIntosh, Klipsch, Marantz, Dahlquist, Sequerra, etc., etc. At one time I had 10 pairs of speakers in my living room (not all runnning at once).
As someone above mentioned - the best results are from pairing speakers that have different strengths and weaknesses. Combining similars seems more problematic with less to be gained.
It's true that careful attention should be paid to time and phase alignment of the midrange drivers relative to your seating position. But it won't be as perfect as a really well put together 3 or 4 way single pair. Somehow, though, the more disparate the speakers' tonal balances, the less phase anomalies seem to have been an issue in my experience.
While purists would scoff, if you want to really get the most flexibility in achieving crazy sound on the cheap with this type of setup, one or two equalizers can be helpful. If you've got one with an accurate analyzer you can still achieve flat freq. response while tweaking the best parts out of each speaker pair.
A couple of actual examples -
I combined Dahlquist DQ-20's with a pair of Sequerra Metronomes gradually taking over the upper mids and highs, which the DQ's are little soft on. The result was the smooth DQ sound, but with more zip in the highs. Imaging was still great because the Metronomes, being small, could be placed in the same vertical "line" as the DQ drivers while maintaining the same distance to listening position.
Combined an early 60's pair of University 12" w/whizzer cones full-range drivers in giant cabinets with Ohm/Walsh 2's (which have great super tweeters). The University's had almost non-existent highs but fast and accurate lows and decent mids. So I ran both pairs practically full range, except to reduce output in the mids of both to achieve flat freq. response (with 2 eq's) . The Ohms were placed to the outside but forward of the Univ's to keep distance to listener constant. The sound was phenomenal - holographic, 3 dimensional imaging and amazing slam. Total cost of the speakers: $80
Listening enjoyment: Priceless!
Combined a pair of Spica TC-50's with (hold your nose) a pair of Bose 501's. Just sat the Spica's right on top of the 501's and ran both full range. Obvious loss of some imaging/detail - but so what. They sounded way, way better than the Bose's alone and significantly better (as in more fun to listen to) than the Spicas alone.
Combined a pair of Dahlquist DQ-20's and DQ-20i's placed side by side. The increase in slam and dynamics was dramatic but there were phase cancellation issues that I couldn't resolve satisfactorily because of the cabinet shape and size, or maybe the oversize grills. I came to the conclusion that they might be a true giant killer with one pair stacked upside down vertically above the other - but to do that right would require welding up a rack to hold the top pair, some cabinet size reductions, and building a new set of grills to hide the whole mess. Impending marriage nixed that experiment.
Combined Spica TC-50's with a pair of Magneplanars (don't remember the model - large single panel) but interestingly, the result was almost no change.