Twin Wands: A Futanari Witch Story

By Machine George. Available on Smashwords.

Every climax is a spell. Every battle is foreplay. When the Rift tore the world open, Rowena was supposed to be the one sent to put the twin magus sisters in chains. Instead, she became their conduit. Now she is bound—body, magic, and will—into a living circuit of sex and power, the only thing standing between the city and oblivion.

WARNING: Contains explicit content. 18+ only.

In the wiki

Stubs on the Futagen wiki (Smashwords title; light spoilers):

Get Twin Wands: A Futanari Witch Story on Smashwords