The first homicide followed a dispute between neighbours in the Ville Saint-Laurent borough. Police say a 39-year-old man was stabbed in a home on Grenet Street at around 7 p.m. Thursday.
He was later pronounced dead in hospital.
Police arrested a 27-year-old suspect whom they say will be questioned and charged on Friday.
Gunshots in Côte-des-Neiges
The second incident happened about four hours later, when gunshots were heard near Kent Park in the Côte-des-Neiges district.
"Police arrived on scene and found the victim lying on the south side of Appleton Street, near Hudson," said Montreal police spokesperson François Collard.
The 20-year-old gunshot victim was pronounced dead in hospital.
Provincial police arrested four men driving in an SUV near the Ville-Marie Expressway several hours later, and Montreal police believe they may be linked with the Appleton Street homicide.
Police say there is no reason to believe the two homicides are related.
Police are asking anyone with information to call Info-Crime at (514) 393-1133.