The Irish 18-year-olds - whose lack of singing talent has sparked criticism of the ITV show - received some of their highest praise from judges following their rendition of Queen's Under Pressure on Saturday night.
But according to The Sun, technicians were ordered to lower the levels on their microphones so their voices were drowned out by a pre-recorded backing track, although show chiefs responded to the allegations by saying all the contestants had "technical support".
The newspaper quoted a programme source who claimed: "Every week session singers record backing tracks to some songs. But soundmen are told to turn down the levels of the twins' mics so their vocals are mostly covered by the backing singers. Often mics are so low the twins' singing would be barely audible."
However, a spokeswoman for production company talkbackThames said all X Factor hopefuls receive an "equal amount" of technical help for their live performances.
She said: "All contestants sing live on Saturday night and all are given an equal amount of technical support to ensure they give the best live performance they can.
"Technical support is standard for all artists when giving live vocal performances."
At the weekend chart act Calvin Harris crashed the stage with a pineapple on his head during John and Edward's performance, later claiming he did it to "make a mockery" of the competition and its "terrible" music.