The 19-year-old singer - who has recently been campaigning to join the panel on 'American Idol' - will help his pal pick from the acts in her category to determine who will go through to the live shows, according to The Hollywood Reporter.
Although it has not been confirmed, it has been suggested Demi will mentor singers aged 12 to 16, while fellow newcomer Britney Spears will take on those between 17 to 24.
L.A. Reid will take those over 25, while Simon Cowell - who left the UK version of the show to begin work on the US adaptation in 2011 - will mentor the groups.
Demi recently revealed she was a ''happy'' judge on the panel and has found it very exciting to take part in the second season of the show.
She said: ''The most fun about being on 'The X Factor' is seeing people that really wow you and knowing that you saw them before they were going to become stars. It really is an exciting feeling.
''I would say that I'm a happy judge. I'm really, really honest. That's the great thing about this panel - we're really honest.''
As they do every summer, the Martin family relocate to their summer home over the...