Brothers of the Head — Brian W. Aldiss

Siamese twins, Barry and Tom, have a dormant but sinister third head growing out of Barry's left shoulder. Plucked from the untamed Norfolk coastline by showbiz entrepreneurs cashing in on their demonically violent and freakish relationship, they form a rock band, The Bang Bang, and taste superstardom. Throughout their jealous battles over the woman in their life and their brutal existence, they are forever accompanied by their ominous companion, who makes increasing demands for its right to life.