The victim, Oliver Tetlow, from Harlesden, was killed in the shooting in Church Road on the night of 9 March. Police believe he was not the target.
A 29-year-old man was arrested on suspicion of murder on Friday in the Shepherd's Bush area.
Earlier in the day, police arrested an 18-year-old in Harlesden on suspicion of robbery. Both men are in custody.
The shooting in Harlesden, at the junction with Colney Road, is being investigated by police as gang-related but Mr Tetlow's death was a "tragic case of mistaken identity", the force said.
The victim was shot multiple times and an automatic weapon appeared to have been used, police said, but the firearm has not been recovered.
Det Ch Insp Mark Lawson said: "The investigation to identify and arrest those responsible for this senseless murder is moving at a very fast pace, and I would like to thank everyone who has so far come forward and spoken to the police."
Police said the killer was in a dark-coloured vehicle which travelled along Church Street towards Harlesden Town Centre after the shooting.
Two men have been arrested after a 27-year-old was shot dead on a north-west London street, Met Police have said.