The Planet Savers by Marion Zimmer Bradley, Science Fiction, Adventure

Desperate to discover a cure for the cyclical 48-year-fever, known as Trailmen's fever, Dr. Randall Forth persuades a colleague, Dr. Jay Allison, to undergo hypnosis. He calls forth a secondary personality, Jason Allison, is gregarious and an experienced mountain climber, while Dr. Jay Allison is a cold, clinical man with no outdoor skills. Jason is asked to lead an expedition into the Hellers to collect medical volunteers from among the Trailmen. Accompanying him are Rafe Scott, Regis Hastur, Kyla Raineach, a Renunciate guide, and several others. During the trip, Jay/Jason yo-yos between his two personalities - one warm and charming, the other distant and clinical. Jason, the warm personality, falls in love with Kyla..... By the time I got myself all the way awake I thought I was alone. I was lying on a leather couch in a bare white room with huge windows, alternate glass-brick and clear glass. Beyond the clear windows was a view of snow-peaked mountains which turned to pale shadows in the glass-brick. Habit and memory fitted names to all these; the bare office, the orange flare of the great sun, the names of the dimming mountains. But beyond a polished glass desk, a man sat watching me. And I had never seen the man before. He was chubby, and not young, and had ginger-colored eyebrows and a fringe of ginger-colored hair around the edges of a forehead which was otherwise quite pink and bald. He was wearing a white uniform coat, and the intertwined caduceus on the pocket and on the sleeve proclaimed him a member of the Medical Service attached to the Civilian HQ of the Terran Trade City. I didn't stop to make all these evaluations consciously, of course. They were just part of my world when I woke up and found it taking shape around me. The familiar mountains, the familiar sun, the strange man. But he spoke to me in a friendly way, as if it were an ordinary thing to find a perfect stranger sprawled out taking a siesta in here. "Could I trouble you to tell me your name?"