The pair spoke only to confirm their names.
Mr Middleton, 21, of Bruntoft Avenue and Mr Sowerby, 23, of Allerton Close, both Hartlepool, are to reappear at Teesside Crown Court on Tuesday, January 7.
No bail application was made and the men were remanded in custody.
Mr Denton was once ranked England's second best amateur middleweight boxer.
Cleveland Police launched a murder inquiry after the 31-year-old died at a party on Hutton Avenue, near the town centre on New Year's Eve.