The Planet Savers. Darkover

Darkover was experiencing a flare-up of trailmen's fever, a disease that would decimate the entire human population of Darkover, from the Comyn to the Terrans. The Medical Branch at Terran HQ had the start of a cure, but in order to finish it, they needed trailmen to come out of their mountain heights and donate blood. Only one man on Darkover stood any chance of persuading the trailmen to help, but he occupied the same body as the doctor capable of doing the medical side of the work, and he was the personality the doctor had utterly suppressed.
Even with hypnosis, only one of them could be active at a time, and the solution would need both of them. This edition contains an introduction by Marion Zimmer Bradley and a foreword by Elisabeth Waters.