Alison McDonagh, 47, and Stephen McParland, 54, absconded while on temporary release from jail.
McParland was sentenced for the murder of a 33-year-old man in 1997, while McDonagh was jailed for the murder of a man in the Rathcoole area of north Belfast in 2004.
Detectives say anyone who sees the pair should not approach them and should contact police.
Officers have also urged the killers to hand themselves in.
The pair were last seen at Lanyon Place train station in Belfast at around 1.45pm on Saturday. Police believe they are still together.
McDonagh is described as being around 5ft 2in and of stocky build with auburn hair and green eyes. She has two tattoos with the words “mum” and “dad” on the underside of her right forearm.
She was last seen wearing a sky blue-coloured puffa jacket and light-coloured leggings, carrying a bag for life and pulling a trolley bag.
McParland is around 5ft 10in and of medium build. He has blue eyes, balding “dirty-fair” hair and a tattoo on his left arm. He was last seen wearing a black hooded coat and grey trousers, pushing a large amount of luggage.
McDonagh was on a pre-release scheme and McParland was on pre-release testing at the time they absconded, the Department of Justice said.
The Police Service of Northern Ireland said anyone who knows where they are should contact police on 101 quoting reference 1972 of 21/1/23.
People can also contact the independent charity Crimestoppers anonymously on 0800 555 111.