Kelly and Paulina will replace Britney Spears and record producer Antonio LA Reid, for the show's third season.
It's a reunion for Kelly and Simon, although American audiences might not know that Kelly was a judge for a season on the UK X Factor.
After the news was announced, Paulina Rubio told Cowell to "be careful what you wish for", adding that she wondered if the notoriously cranky judge would be ready to handle her.
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Read the fine print on a contract. If you're not happy with the proposed terms of a deal, renegotiate. Don't be afraid to drive a hard bargain. You may not realise it, but you have a great deal of power. People are willing to pay good money for someone with your intelligence and experience. If you undersell yourself, you will set a dangerous precedent. Someone will ask you to do something that goes against your conscience. Obey your principles, even if it means making an enemy.More