A Quincy man pleaded not guilty Friday to two counts of murder in the shooting deaths of Victor Otoadese, 21, and Sean Repetto, 25, in Dorchester, said a spokeswoman for the Suffolk district attorney’s office. Prosecutors said Keon Monteiro, 24, shot Otoadese and Repetto on Batchelder Street on the afternoon of Nov. 28, 2012. Prosecutors said that the victims were seen with Monteiro shortly before they were shot and that witnesses identified Monteiro, telling police they had seen him leaving the area with a gun in his hand. Both victims were taken to area hospitals, where Otoadese died shortly afterward, said Renee Nadeau Algarin, spokeswoman for the Suffolk district attorney. Repetto succumbed to his injuries days later, she added.