Police were called to West Harrow Park shortly after 8.34pm on Saturday, 7 August to reports of a stabbing.
Officers attended along with paramedics from London Ambulance Service.
A teenage boy, believed to be 17, was found with stab wounds.
He was treated at the scene by paramedics before being taken to the hospital.
His condition has been assessed as non-life-threatening.
A short time later another teenage boy, believed to be 15, self-presented at the hospital with stab wounds.
He is currently in a life-threatening condition.
A crime scene has been put in place in West Harrow Park.
There have been no arrests.
Enquiries are ongoing.