Police were called just minutes apart to the shootings at two locations in close proximity in High Street, Wealdstone.
Both boys have been taken to hospital and Scotland Yard said they await details of their condition.
Officers were called to High Street at 1.17pm on Sunday to find a 15-year-old boy had been shot.
-> VIDEO: Marshal leaps out of way of Tour car in narrow escape on final day of Tour de YorkshireTwo minutes later they were alerted by London Ambulance Service about a second victim, aged 12, who had been shot at a different location nearby.
Police said they are investigating whether the two incidents are linked.
No-one has been arrested and no firearm has been recovered.
Scotland Yard said officers were called to High Street in Wealdstone at 1.17pm.
A 15-year-old boy was found with a gun shot wound.
Two minutes later at a different but nearby location in High Street, a 12-year-old boy was also treated for gunshot wounds.
Both boys were taken to hospital and police said they await details of their condition.
No arrests have been made and no firearm has been recovered, police said.