The music mogul tweeted a pic of himself and the ladies with fellow male judge, record boss LA Reid.
Minutes after posting the photo, the quartet attended an official presentation in New York.
Britney, who arrived in a white mini-dress but changed into a burgundy one for the official photos, said: "I'm so excited by this whole experience. It's so different from anything I've ever done."
Simon said of the...Baby One More Time singer – who found fame with the track aged 16: "Britney remains one of the biggest stars in the world, she's talented, fascinating - and I believe she knows exactly how to spot the x factor."
He was equally complimentary about 19-year-old Demi, first seen on Disney show Sonny With a Chance before she launched herself as a pop singer.
"Demi's had an amazing career in music, TV and film for someone her age. She's young, confident, and enthusiastic. I think it's really important that she speaks to our younger audience," he said.
Details of the judges salaries are not known, though Briney is rumoured to be pocketing $15 million a year for doing the show – making her one of the most highly-paid reality TV judges.
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