The incident happened at around 7.30pm on Friday 22 October 2021 in Revenge Road and involved a white Honda 125 motorcycle and a black off-road motorbike.
The rider of the Honda, a man aged 19, suffered life-threatening injuries following the collision and the rider of the off-road motorbike, a boy aged 16, suffered serious injuries.
Both riders were taken to a London hospital where they remain.
Officers from the Serious Collision Investigation Unit (SCIU) are continuing to investigate and are appealing for witnesses to come forward.
In particular they would like to talk to a group of around six young people who are thought to have seen the collision but have not yet spoken to officers.
Witnesses with dash cam or mobile phone footage showing the incident are urged to make immediate contact with officers.
Anyone with information should call the SCIU appeal line on 01622 798538 or email email@example.com quoting reference DS/118/21.