Michael McCormack, 59, of Ewell, Surrey, and John Ogden, 34, of New Malden, Surrey, were shot by a gunman who was lying in wait as they walked along Cavendish Road, Balham, on Friday afternoon. Both were married and had children.
Mr McCormack, a father of two, was a director of The Indexing Company, a printing and bookbinding business based on an industrial estate a few minutes' walk from the scene of the murder. Mr Ogden was the works manager.
He was married with a young daughter.
Police said that two of Mr McCormack's businesses had 'gone bust', but he was believed recently to have weathered his financial difficulties.
Police have been examining business files belonging to the two men in the hunt for possible clues, but a senior officer in the investigating team said nothing had turned up 'that would give even a hint to possible motive'.
A businessman whose firm is on the same industrial estate in Zennor Road, Balham, described Mr McCormack and Mr Ogden as 'very nice men'. He said Mr McCormack, who he had known for 14 years, was 'very easy-going' sort of man.
One officer working on the murder case said: 'We cannot at the moment rule anything out. However the chances of the killer being known to the men or to locals is unlikely given the openness and public nature of the killing. But anything is possible.'
Mr McCormack and Mr Ogden regularly lunched together at the Prince of Wales pub opposite the sandwich shop where their killer lay in wait.
Police said the gunman had been eating a sandwich and drinking tea inside the shop when the pair walked by. He jumped up from his seat, walked outside and shot both men dead at close range.
He made his escape in a white Vauxhall Cavalier - registration D880 MYG - which was later found burnt out in Herne Hill.
Anne Vanoli, landlady of the Prince of Wales, described Mr McCormack and Mr Ogden as friendly and polite. She said they had been nicknamed 'cheese' and 'cheese and onion' because of the sandwiches they ordered regularly.
Detective Inspector Adrian Smale, one of the police hunting the killer, said the post- mortem examination revealed Mr McCormack was hit first with a single shot to the back of the head with a heavy calibre handgun. Mr Ogden must have turned round when he heard the shot. As he turned, he too was killed with a single shot.
Det Insp Smale said the murders were a complete mystery. 'Both men appear to have been ordinary hard-working family men. Their families are totally devastated, bemused and bewildered.'
He said police were looking into their backgrounds and speaking to family members and other people who knew them. They were also conducting house-to-house inquiries in the area of both their homes and the business premises.