Both men are understood to have had maps and aerial photos of her rural property in east Devon.
They were initially arrested on Monday morning on suspicion of having an offensive weapon and planning a burglary.
But Sky News understands they are now being held on conspiracy to rob and murder Stone, 24, who is said to have a £9m fortune.
Her neighbours are said to have foiled the plot when they saw two men acting suspiciously in a red Fiat Punto near her property in the Cullompton area.
Stone, a friend of Prince William and Kate Middleton and a guest at their wedding, is not believed to have been home at the time.
Her spokeswoman said she was aware of the arrests and police were keeping her updated on the developments.
Earlier a white sports car left the front gate of her house with someone covered by a blanket in the first passenger seat. It is not clear if it was Stone.
Detective Inspector Steve Parker, leading the investigation, said: "The two men in custody had in their possession information relating to an individual in the Cullompton area and items which lead us to suspect that they may have intended to commit a criminal offence.
"We're unable to say anymore at this time but I would like to thank the local residents for reporting the suspicious car."
Stone, who has sold more than 10 million albums, is due to go on a European tour in a few weeks.
Source : http://news.sky.com
