[Wayne A. Pflughaupt]

1. Hmm… never heard that one before, but any “ping ponging” will be from the soundwaves of the speakers bouncing off the walls. Whether or not they’re aimed at each other has no bearing on that. The diffused soundfield (from direct radiating speakers at least) has to do with a lot of factors, but the most relevant ones are probably how wide of a dispersion pattern the tweeters have and your physical distance from the speaker.

2. That will depend on how close they are behind the listener, and whether or not they’re oriented towards the seating. Obviously the further the better. When they get that close, vertical distance matters as much as horizontal. I'd say probably at least 2-3 feet behind for a hard 90-degree placement (i.e., with the speakers directly facing each other), and double or more that if they're angled towards the seating. Less that that it's hard for a listener close to one speaker to not be overwhelmed by it to the exclusion of the other.

Regards,
Wayne