The fifth head of Cerberus

From the back cover Ace Books paperback September 1981:

Sainte Croix and Sainte Anne... sister planets where the lives of humans and aliens merge like dreams and twist into nightmare. A haunting, lyrical, finally chilling triptych:

-- John March, Terran anthropologist, searches for the truth about the vanished aboriginal culture of the sister planets.

-- Sandwalker, an abo child, searches for Eastwind, the long-lost twin brother he has met only in dreams.

-- Number 5, reared in a Sainte Croix brothel, searches for his identity in a world of genetically engineered aliens clothed in human bodies -- and human memories.