The Syco boss tweeted this weekend about his return to the show following illness, saying that he had noticed a change in both of the female panellists during his absence.
"Unfortunately Britney & Demi are now known as the gruesome twosome," he jested. "V happy to be back-Demi more annoying than ever, Britney has grown claws!"
Cowell later revealed that the X Factor judges have discovered some promising acts during the latest rounds of auditions.
"In San Francisco - just seen an incredible group. Watch out for them! @TheXFactorUSA," he wrote. "One of the best audition days we've ever done - stars have come to San Francisco!"
Cowell was replaced by X Factor UK veteran Louis Walsh while off sick. The Irish music manager also received light-hearted teasing from Spears and Lovato during his brief appearance.
Cowell recently defended Spears against suggestions that she has displayed "unstable" behaviour on the show.
"She's working out great," he said. "I mean, all these reports that we've fallen out, it's nonsense. She's doing a great job.
"Once people give her a chance, I think people are really going to get her. She's gonna be a big surprise. She's a tough judge."