When my wife and I ride rail trails together, I play lite music off my mp3 player through two powerful little speakers I purchased on eBay. One is on my handlebars; the other is mounted to my rear rack. Although each speaker puts out a good amount of power when used alone, I lose quite a bit when I piggyback them. So when she lags behind by more than 15 feet or so, she can't really hear the music anymore.
Music is a big part of our rides, so I'd like to find some kind of Bluetooth system that will transmit between my bike and hers at distances of 30-40 feet, if this is possible. The music would have to play through my speaker and her speaker simultaneously, and sound quality is pretty important.