Changewinds #2: Riders of the Winds by Jack L. Chalker

When the Changewinds blow from the Seat of Probability across the worlds they themselves created, they are at once random and consistent, obeying their own spectral meteorology. Most of the universes created by the Winds exist separately in time and space, but close to the Seat of Probability they are more densely packed; one may walk from one universe to another and not realize it until too late….
Over this tumultuous sea of realities rule the Ahkbreed, whose sorcerers have mastery over physical laws, giving them absolute power over an empire of worlds beyond counting.

For countless centuries those who pay tribute to the Ahkbreed have dreamed of a deliverer. For countless centuries, the Ahkbreed sorcerers have dreamed of the ultimate power — control and direction of the Changewinds themselves, a power that would truly make them gods over all universes everywhere. And now two brave young women from Earth have been caught up in the fury of the Changewinds.

They will themselves be transformed, but in the process they will Change everything.