Depends if it's for home use or stage/studio use.
For home use, I'd say put them as far apart as you like so that you like the sound, you could get a neat sound from picking two opposite corners of a room.
For stage use, it doesn't really matter, just mic them and put them to left and right sides of the p.a. at the mixing stage.
For studio use, again, most of the work is done in the mixing stage, but you'll probably want them pretty far apart to avoid one interfering with the other's sound. Like, even to the point of separate rooms if you feel like going all out.
Basically, whatever sounds best. Just watch out:
If both speakers are running the same sound at the same time exactly (like, no stereo effect, just mono), you get the insane interference problem that physics people love to play with. We had to do it in Physics class, and even had it demonstrated with actual speakers and a signal generated that just created a pure tone. As you walk across the classroom, you get varied areas of loud and soft noise. Because of the wave patterns, you get areas where the waves support each other, and areas where they cancel each other out. The noise was annoying, I found a quiet area in the wave pattern: behind a large cement pole used for holding the ceiling up.
Hey, so I cheated a little.