Mr Cassidy separately is also accused of more than 40 offences over July and August, including alleged car thefts, aggravated burglary, criminal damage, deception and arson.
Mr Cassidy will appear at Melbourne Magistrates Court on Thursday.
Byron Hampton, 16, and Jordy Kirkwood, 18, died when the car they were riding in allegedly crashed into another car at 4pm on Sunday on the South Gippsland Highway.
Another passenger, 17-year-old Dakoda Nicholson, was fighting for her life this week. Her condition improved on Wednesday to serious.
A friend said only family had been allowed to sit by the Frankston North girl’s bedside at the Royal Melbourne Hospital.
Baxter man Jack Ledlin, 18, was charged on Tuesday with allegedly stealing the car the group was riding in.
He was a passenger in the car and had been treated for minor injuries at Dandenong Hospital.
The occupants of the other vehicle – a 39-year-old woman and two girls aged six and nine, all from Lynbrook – were treated for non-life threatening injuries.
Anyone who witnessed the incident of who may have dash cam footage is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at www.crimestoppersvic.com.au.