32-year-old left-back Newey is well known to manager Chris Wilder from their time together at Oxford United and a short loan with the Cobblers earlier this season which was cut short by injury.
"He was superb for me at Oxford, we had a great defensive record there and he was a part of that.," Wilder told BBC Radio Northampton after sealing a deal with the player who was released by the U's earlier this week.
Portsmouth winger Holmes, 27, moves on a free transfer after impressing on loan at Sixfields in recent weeks. He has signed a two-and-a-half year deal.