One year after Mighty Mouse made his television debut, Terrytoons stablemates Heckle & Jeckle joined him on CBS' Saturday schedule. However, the "Talking Magpies" didn't have the staying power of the "Mouse of Tomorrow", and were cancelled after 1 season, only to be brought back in 1965 when CBS had a hole in the schedule that needed to be filled. This time, they lasted two years before being swept out again. NBC picked up Heckle & Jeckle for a 2 year run from 1969-71, and then the theatrical shorts went into syndication, coupled with a new set of cartoons made for the later CBS & NBC runs.
To distinguish one from the other, Heckle speaks with a Brooklyn accent, while Jeckle is British in background.
I first discovered Heckle & Jeckle during their syndicated run in the 70's, when it aired on WPIX in New York & WSBK in Boston. In 1979, the magpies returned to CBS, this time as part of a 1 hour block with Mighty Mouse. This time, Frank Welker (Scooby-Doo) voiced both characters in Filmation's incarnation, which didn't last very long, either. Right now, I can't find any of the Filmation toons, but in the meantime, here's the color version of the open for the Terrytoons show, uploaded by Muttley16 to YouTube:
CBS-Paramount owns the rights, but don't believe putting the classic toons on Nickelodeon or Nicktoons will create a new generation of fans for the magpies. Their loss.