The embattled Upper Crust pizza chain, already under federal investigation into its labor practices, is now riven by a nasty battle for control of the company among its co-owners, who are accusing each other of misusing corporate funds for personal extravagances. The fight is spelled out in a complaint and counterclaim filed in Suffolk Superior Court by Upper Crust co-owners Joshua Huggard and Brendan Higgins on one side, and founder Jordan Tobins on the other. In April Huggard and Higgins sued Tobins, accusing him of “looting” the Boston pizza chain by charging more than $750,000 in personal expenses to the company, including buying a Cessna airplane, and asking the court to force Tobins to surrender his stakes in Upper Crust and its related businesses.