I'm trying to identify a novel from the mid 1990s that was based around the idea that a certain subset of the population lived a double life. When they slept they existed in a medieval world and when awake they lived in our current world.
The protagonist was under the impression that he was the only one living this double life. Eventually he was discovered by the others who had this power. As it turns out those that lead this double life had some magical abilities. Their powers were in pairs: life/death, water/fire that sort of thing. The protagonist is able to manipulate life and death.
Some adventures ensue, the details of which I cannot remember. I do remember that in the end of the book the protagonist saves somebody's life by taking the life force of his own unborn twins.
It always felt like a book crying out for a sequel but I never found one and I've since lost the name of the book.