The 18-year-old and a boy, aged 17, are alleged to have been riding a stolen off-road motorcycle early on Monday when it was involved in a crash with a four-wheel drive in the Perth suburb of Hillarys.
Both were rushed to hospital with serious injuries, and the 18-year-old remained in a critical condition on Wednesday.
The 17-year-old has been charged with aggravated home burglary and stealing over the alleged theft of two off-road motorcycles from a nearby home earlier that morning.
He faced court on Tuesday for a bedside hearing.
The driver of the four-wheel drive, a 49-year-old woman, also suffered serious injuries after her vehicle rolled and caught fire.
Police have interviewed the woman, who is believed to live at the home from where the motorcycles were stolen. Detectives are investigating whether she had been chasing the youths in her car when the crash happened.
WA Police Commissioner Col Blanch said the investigation was ongoing and asked anyone with CCTV or dashcam footage to come forward.
"Clearly there is allegations that they were being chased ... we haven't formed any of those outcomes or findings yet," he told ABC radio.
While declining to comment further on the Hillarys investigation, Mr Blanch urged people who may be victims of crime not to take vigilante action.
Parallels have been drawn with the 2016 death of Kalgoorlie teenager Elijah Doughty, who was riding a stolen dirt bike when he was run down and killed by its owner.
The man was charged but later acquitted of manslaughter, before being convicted of dangerous driving causing death and serving half of a three-year jail term.
"We've seen it many, many times, people taking chase of offenders when they've witnessed a crime," Mr Blanch said.
"Consider your circumstances before you take things into your own hands because it can end in tragedy for yourself, other members of the community or the person that you might be chasing.
"No one's life is worth more than property."
