Actually, this had occurred to me as well, independently. ATC primary radar and a lot of surveillance radar have about 1 microseconds = 150 meters ~ 500 feet (two way ranging time) resolution. Two aircraft separated by less than 500 feet in range, will look like one aircraft to the radar. The primary radar is the only way to detect an aircraft without a transponder. Flying 500 feed directly below and slightly behind the other aircraft, he’d probably be undetectable.
Interesting but wouldn’t the plane be noticed by the one in front? Do commercial planes have “rear vision” video? Or would the aero be affecting the plane in front?