William Lauchlan, 35, and Charles O'Neill, 51, are both serving life sentences. Lauchlan cannot apply for parole until he has served 26 years and O'Neill must serve 30 before he can ask to be freed.
They dumped the body of victim Alison McGarrigle, 39, at sea after the murder at Largs, Ayrshire, in June 1997. Her remains have never been found.
Judges at the Court of Criminal Appeal in Edinburgh Lord Carloway, sitting with Lords Menzies and Brodie, were told that the sentences were out of line with other cases where no cause of death had been established, but rejected the argument. The two men had earlier tried, without success, to challenge the murder convictions.