Richard Avery is accused of being part of the "planning and preparation" of the murders of Will Henry, 31, and Brian McIntosh, 29, at the Albion Works industrial estate in Brierley Hill last September.
Jonathan Houseman, 32, is accused of shooting dead both men over a £200,000 debt he owed them.
Jurors at Birmingham Crown Court were told how Avery's DNA was detected on cartridge casings found in the bloodstained Range Rover following the execution-style murders.
The 33-year-old ran a valeting service at the Merry Hill shopping centre and knew Houseman as he had cleaned cars for him.
Forensic scientist Andrew Davidson told the court his tests showed it was likely Avery had handled the cartridges.
Answering questions about a Glock pistol bullet casing found in the boot of the Range Rover, he told the court: "In my opinion there is strong support for the view Richard Avery touched the casing rather than he did not."
Avery, of no fixed address, who denies murder and perverting the course of justice, claimed in his defence statement that he had previously valeted the Range Rover and that is how his DNA must have got onto the casings.
But Mr Davidson said if that had been the case he would have expected to have found higher traces of DNA from other people, including the car's owner Mr McIntosh, inside the vehicle rather than someone who had merely cleaned it.
He added he thought it unlikely DNA could have transferred to the casings without being physically touched.
"I would expect to see DNA from regular users of it and a higher level than someone who had simply valeted it," Mr Davidson said.
Friends and business partners Mr Henry and Mr McIntosh, both fathers from Bartley Green, Birmingham, were found dead in the car on September 30. Both knew alleged shooter Houseman and had previously worked with him.
Houseman, of no fixed address but formerly of Quarry Park Road, Stourbridge, denies murder.
Avery’s partner, 33-year-old Francesca Scott, of Lower Valley Road, Brierley Hill, denies perverting the course of justice.
The trial continues.