The youngster was hit by the shotgun pellets in Harrow as a 15-year-old was attacked at around 1.15pm on Sunday in High Street, Wealdstone.
Scotland Yard said the older boy was found to have suffered head injuries, caused by a number of shotgun pellets.
Minutes later, police were alerted to the second boy injured nearby.
Detective Chief Superintendent Simon Rose, Harrow borough commander, said: "It would appear that the first victim was approached by two male suspects, one in possession of a shotgun - and shots were fired, injuring him and an entirely innocent member of the public.
"This was a callous, reckless and brazen act, without any thought by those responsible for the fact that there were families with children and people in the High Street enjoying their weekend. This was quite simply appalling."
Both teenagers suffered non-life threatening injuries, and the younger one has been released from hospital.
Officers have also been made aware of a possible third victim who may have been at the scene with the 15-year-old boy.
It is thought he sustained an arm injury and left prior to the arrival of the emergency services.
A 39-year-old man arrested on the evening of May 6 in connection with the incident has been released under investigation.
Extra police officers, supported by armed units, are patrolling the streets of London in a bid to combat the bank holiday violence.
Rhyhiem Ainsworth Barton was fatally gunned down in Southwark, less than a day before the Harrow shootings.
The 17-year-old rapper and aspiring architect had "so much potential", his mother said as she tearfully told of her "handsome boy".
Police have said extra patrols will be working on Bank Holiday Monday to keep the capital's streets safe.
Detective Chief Superintendent Simon Messinger said: "The violence used has rightly caused concern and we are doing all we can to address this.
"Over the bank holiday weekend, additional officers are on the streets across the capital, working hard to keep London safe.
"They are using highly visible local patrols supported by armed response, traffic motorcycles, dog units and air support, alongside plain clothes intelligence-led operations.
"Violence has no place on our streets."
Rhyhiem's death is the latest in a spate of violent crimes in the capital, as police investigate more than 60 alleged murders so far this year.
Official statistics released in April showed the number of homicides in London had surged by 44% in the last year.
Mr Messinger said officers were using search powers to seize knives and to stop weapons being carried in cars.