The judge will give the operatic singing duo a £1m Syco deal regardless of tonight's (May 12) final result, The Sun reports.
Meanwhile, Sony boss Rob Stringer is said to be flying in from America to watch the pair perform and plans to offer them a US deal with Columbia Records.
"Simon knows they will sell a huge number of records. They could even go on to beat Susan Boyle - and she's sold millions," a source said.
"This time last year he was faced with the not-particularly exciting prospect of having to sell Jai McDowall records. He was in a funk. But this year his excitement levels are off the scale.
"He's watched as Jonathan and Charlotte have taken the country by storm. He knows that opera is a big seller, especially if people like the person singing it."
Jonathan and Charlotte will start recording in the studio the week after next. An album is expected to be released by Christmas.
Showbiz agent Jonathan Shalit, who masterminded the careers of Tulisa Contostavlos and Myleene Klass, will manage the duo.
Cowell said: "I think they've got a big, big chance of selling records all over the world. They're amazing singers. I'm particularly impressed with how Charlotte has improved."