Dimitry, I asked this same question a while ago (Long term biphasic sleeping
); however I wasn't exactly inundated with responses from long-term adherents to either biphasic or polyphasic sleeping patterns. Personally, I've been biphasic for a little over two months, and there's no temptation at all to return to monophasic. I'm not quite sure why you'd want to mix the two.
Certainly in the case of biphasic (and also in polyphasic, from what I've heard) the quality of sleep increases. In fact, there are several benefits quite apart from the time reduction. To get an idea, take a look at the summary
of my initial 30 day trial.