I've successfully rooted a few lucky bamboos and cared for one specimen which grew to huge heights and lasted for many years! While I have never personally rooted horizontally, I don't see why it couldn't work. Typically whatever portion of the plant is in soil/water will begin rooting--it should work horizontally or vertically.

Remember to use a clean, sterile knife when cutting and make sure to use clean, quality soil as well. I'm not sure about anyone else, but many of the lucky bamboo specimens I've had were very susceptible to rot!