Staff Insp. Greg McLane said Nahom Tseigazab, 19, has also been charged with 23 counts of aggravated assault.
Numerous gun shots were fired during a July 16 barbecue for young people, killing fourteen-year-old Shyanne Charles and 23-year-old Joshua Yasay. Twenty-three others were sent to the hospital, including Tsegazab, who was also shot.
McLane said Tsegazab appeared in court Thursday but did not enter a plea.
Shaquan Mesquito, 18, was the first person charged in the case two weeks ago. He faces two counts of first-degree murder charges, attempted murder and 23 counts of aggravated assault.
McLane said an altercation at the barbecue, with up to 150 people in attendance, touched off the deadly violence between rival members of the Galloway Boys and Malvern Crew gangs.
He said police believe one or two other shooters may have been involved. Police recovered more than 25 fired shell casings and five firearms at the scene.
Police have linked the Galloway Boys to five other shootings in eastern Toronto between Sept. 4, 2011 and Aug. 10, 2012.
Mass shootings are rare in Canada but a series of attacks in Toronto and other cities over the summer raised fears that the country could be moving closer to U.S. levels of gun violence despite its stricter gun control laws.