Before digital effects could place an actor's face on his body double - like in THE SOCIAL NETWORK where Armie Hammer plays the Winklevoss Twins - split screen effects were the best a twin sequence could get. An actor was required to play the star's twin via back of the head shots. 
Some of the more famous body double celebrities who gave good head that I can think of -- are Evelyn Moriarty, once Marilyn's THE MISFITS stand-in, who later often doubled for Jeannie's evil twin sister Jeannie II.
here is Barbara & Barbara (split screen)
here is Evelyn and Barb
Bewitched's Elizabeth Montgomery's stand-in was Melody McCord. Melody played the back of lookalike cousin Serena - or Samantha's rear view as needed. Sometimes the director just said "eh, close enough for 2 seconds" and let Melody's face be shown in the final product.
Like Evelyn Moriarty who made a career as stand-in/body double going from MM to Barbara Eden...Melody McCord went from her Extras Guild job with Liz Montgomery to working with Goldie Hawn. She eventually showed her real face on Falcon Crest as Melody the maid.
Finally, there is Susan Henning who signed a Disney contract promising to never tell anyone she was in the The Parent Trap. Years later, Henning told her tale on a special featurette in the Parent Trap DVD. Susan helped Hayley Mills adopt some American teen slang during their time together.
Back to this year's Twin body double star...Armie Hammer was paired with tall model Josh Pence (see below) to play his other half.
