In 1949 on Earth-Two and 1963 (or thereabouts) on Earth-One, the Fastest Men Alive on two worlds encountered their most viable threat. They had built careers on the premise that they could outrun, and out-think, their opponents at superspeed. Only now, their latest foes had their superspeed!
Professor Adrian Zoom of the 24th century journeyed back to the 20th century to duplicate...and surpass...the success of the Flash. Using a process whereby he extracted the hero's powers from his costume, Zoom adopted the alias of the Reverse Flash, with the intent on stealing the love of his foe' Barry Allen's greatest love Iris West! Doctor Edward Clariss was a scientist at the university where a young Jay Garrick gained his superpowers, and when he was scoffed for his theory on the basis of said powers he fled the country. Returning years later having perfected a temporary means to replicate the superspeed, he and his gang of crooks used this speed to terrorize Keystone City. Ultimately he was defeated.
Both men ended up in reported final clashes with the Flashes whereby Zoom was accidentally killed when his Flash prevented him from attacking Barry's new fiancee, and Clariss was consumed by the same force of speed that propelled him towards his former student, Jay.