There should be a mini-catalog with an article "how to get the most out of your paraliminal." PhotoRead it, it's good. There are also tape-specific instructions on the case cards, and in the recordings themselves.
"Float down the middle" is very difficult to explain. For best results, what I've done is follow my intuition as to which voice I'm going to listen to, and then the next time through listen to the other voice. This is like "prepare" in the PhotoReading step. If you listen to both voices one-at-a-time, you have a good idea about what's being said the third time through, and can more easily stay in the middle knowing where the edges are.
While in the middle, you may have an "ah, I hear" experience where the two voices overlap into new phrases. I can't recall any specific examples. It's not overt, so it's not annoying. I'm pretty sure it's intentional. If you really want to hear the messages, listen with headphones plugged into a mono-output.
Now, keep in mind Paraliminal tapes are actually not hypnotic in nature. Rather, they're trance-breakers. While listening to them, you may be physically relaxed, and "drifting" mentally, but you're not entranced or hypnotised.
If you're really interested in the theoretics, and how to use the research that went into these paraliminals for maximum gain, then the Learning Courses are probably good investments.