Henry Kuttner 
The Dark World 

Airman Edward Bond discovers that he shares his body with an alternate version of himself, a despotic wizard named Ganelon. Traveling through a portal into the fantastical alternate dimension, he finds himself embroiled in an epic fight for Ganelon. Trapped between two opposing sides, he finds he must battle not only for the supremacy of his shared mind—but the fate of a world!