Sorry zip, I know there are a couple of good handbooks out there, but not the exact names. Try searching AC or Amazon. Maybe the Junior audiophile or Starting circles would have that information (if not they should). Or try asking one of the AC speaker builders.
Higher frequencies is what the ear senses direction from as lower frequencies are omni-directional. (Sound waves wrap around objects in proportion to their wavelength.) So two tweeters would blur the imaging (like seeing crossed-eyed). Also the energy output at higher frequencies is very low compared to bass frequencies (that's why woofers are bigger). As open baffle is the least efficient way to reinforce bass frequencies, the demand on woofers is increased all the more, so some resort to using additional drivers. Also distortion is reduced when excursion (driver movement in and out) is limited.
