The pair worked alongside one another on the hit Fox series from 2002 to 2009, and got back together in 2011 for the inaugural season of "The X Factor (US)".
The "Straight Up" singer is set to return to TV, joining Simon and record executive Louis Walsh on the panel of "The X Factor: All Stars" - which is set to air later this year.
An insider told Britain's The Sun newspaper: "Paula is a close friend of Simon's and they've always kept in touch since working together in America."
"Paula and Simon have great on-screen chemistry and their banter is funny and entertaining," they continued. "Simon feels comfortable sitting next to her and knows they'll have a laugh and wind each other up, which will make for electric and entertaining viewing."
The duo recently reunited with third Idol judge Randy Jackson on "The Kelly Clarkson Show" earlier this month (September), where the music mogul confessed his "one wish" was to work with his former co-stars again.
"We had so much fun," he recalled of their time working together.