Targets have included a Big Daddy's Pizza on Sunday night.
At another location, the station reported, a clerk says he was forced to open the register with a gun to his head.
Denver police spokesman Sonny Jackson said it could just be a matter of time before someone is seriously hurt.
"We're highly concerned because they are armed, they come in, they're very aggressive and their actions have us greatly concerned," Jackson told CBS Denver.
One suspect was described as being a white or Hispanic man about 5-foot-8 to 5-foot-10, usually wearing a gray hoodie and gray mask or a black leather jacket. The other was described as being a black man about 6 feet tall, wearing a long black wig, a la Milli Vanilli.