The twins, who have built up a huge following on the back of their chaotic performances, have yet to face the judges' vote - much to Simon's annoyance.
He said: "If they win, it will be a complete and utter disaster - I'd probably sulk for about six months, and I'd probably get on a very fast plane out of the country."
Speaking at a press conference to announce his patronage of the Children's Hospices UK charity, Simon said it was impossible to predict the winner of this year's competition - but that he knew who he did not want to win.
"I'm just praying it's not John and Edward," he said. "To be fair, we put them in a competition where it's a public vote, and we have to live, unfortunately, with the public vote."
He said the 18-year-old duo's mentor Louis Walsh would be "in trouble" if the pair were to triumph.
"The fact is, it's a singing competition, and they can't sing," he added. "I don't dislike them as people. I just don't think they are the act we were looking for."