I heard the exact same thing as "Oneiro" about the double fixation from several individuals(including some of Carlos Castaneda's books). I tried it myself and it resulted in two different ways. When I started having lucid dreams again, I tried lying down and trying to go to sleep in the lucid dream, the first couple times I just woke up. After around three tries, I thought I had just awakened, but in fact I just had a false awkening. One particular night, I had three false awakenings in a row after trying to go to sleep in the first lucid dream. When I first started lucid dreaming, false awakenings would always get the best of me, when I would finally wake up it felt like I wasted my sleep on a non-lucid dreaming when the whole time I thought I was awake, I had just falsed awakened. The whole thing about double fixation is something I believe should be practiced after keeping a consistent diary of every dream, or after the expectation of every dream is going to be a lucid dream settles in. I find the double fixation technique of increasing vividness requires great focus, and the ability of recognizing the "feeling" of a dream to notice the difference in a false awakening, and a true awakening. If you can handle spinning in a Lucid Dream to start a new dream, you most likely can handle double fixation.

-J