Prabjot Singh Rayat, owner of Rayat Autos, in east London, was found dead on June 8 on a small industrial estate where the garage is located.
A post-mortem at the Greenwich Mortuary the next day gave the cause of Rayat's death as a stab wound, the police said.
A 34-year-old man and a 44-year-old woman have been taken into custody at south London police stations.
An incident room has been opened at Lewisham police station under Detective Chief Inspector Mark Dunne from the Homicide and Serious Crime Command.
Bartimaeus Clarke, who works in a nearby company, said he was stunned when he came to work and saw the police: "I was shocked when I heard.
"He was quite a happy man and I don't think he had enemies I had worked here for five years and he was always a friendly guy and would say hello."
