My Brother's Keeper — Charles Sheffield

TIME-SHARING TWINS

Lionel Salkind was a rising musical star.. His twin brother, Leo Foss, was a researcher in government work that he couldn't talk about. Then the helicopter they were flying crashed.

When he woke up, Lionel learned that both he and Leo had sustained fatal injuries, and he was only alive because the surgeon had used organs from Leo to repair Lionel's slightly less damaged body. More than half of Lionel's brain was gone, and had been replaced with Leo's. Lionel, in fact, had become HIS BROTHER'S KEEPER