Robert Silverberg 
Second Trip [EPUB ebook] 

Manhattan 2012: Nat Hamlin’s brilliant career as an artist came to an end the day he went insane and embarked on a murderous rampage … his sentence: Total Personality Replacement. Lissa loved Nat for his passion, now she loves him again – but as Paul Macy – for his warmth and kindness. Now each personality wants her help in battling the other, for with her Power, the man she chooses can kill the other. Lissa is terrified. She has to send one of her loves to his destruction. If she chooses the wrong man, the horror will never end.