A man who pleaded guilty as a teenager to the 2001 stabbing deaths of two married Dartmouth College professors is challenging his life-without-parole sentence, saying that the New Hampshire Constitution prohibits it. Robert Tulloch was 17 when he was part of a conspiracy with a friend that resulted in the deaths of Half Zantop and Susanne Zantop in Hanover. He is now 41. A hearing was held Wednesday to consider legal issues raised in Tulloch’s case. At least 28 states have banned life-without-parole sentences for crimes committed when the defendant is a child. Efforts to pass similar legislation in New Hampshire have not succeeded.