The first slaying occurred at about 9 p.m. Tuesday in the 6900 block of South Princeton Avenue in the Englewood neighborhood, police said.
Coleman had been riding a bicycle when, according to a witness, a gunman inside of a tan or gray vehicle opened fire, striking him at least once, police said.
Coleman, of the 13000 block of South Evans Avenue, was taken to the University of Chicago Medical Center and pronounced dead there, according to the Cook County medical examiner's office.
Police were investigating whether the shooting was sparked by a gang conflict in the area, authorities said.
About an hour and a half later, a 19-year-old man was shot on the front porch of a home in the 8100 block of South Ingleside Avenue in the East Chatham neighborhood, according to police.
Muhammed Kebbeh was talking with his girlfriend and other people when two gunmen wearing shirts across their faces approached and shot him numerous times, police said.
Kebbeh, of the 5800 block of South Indiana Avenue, was taken to Advocate Christ Medical Center in Oak Lawn, where he was pronounced dead this morning, a medical examiner's spokeswoman said.
The victim had past ties to street gangs, but they hadn't yet established a motive, police said.
Autopsies performed Wednesday confirmed that both victims died from gunshots and that their deaths were homicides.
No arrests had been made in either shooting.