The original poster suggested we might beat 11; so far we're up to 26 (29, if you count the three iffy actors):
Harrison Ford: Star Wars (3); Indiana Jones (4)
Clint Eastwood: Dollars Films (3); Dirty Harry (5)
Talia Shire: Rocky (5), Godfather (3)
Sylvester Stallone: Rocky (6), Rambo (4)
Arnold Schwarzenegger: Conan (3, counting Kalidor in Red Sonja), Terminator (3)
Orlando Bloom is in LotR and Pirates.
Hugo Weaving is in LotR and Matrix.
Ian McKellen is in LotR and X-Men.
Warwick Davis: Harry Potter (6) Leprechaun (6, including a few direct to video)
Mike Myers - Shrek & Austin Powers
Eddie Murphy - Shrek & Beverly Hills Cop
Matt Damon in Bourne and Oceans.
Mel Gibson - Mad max and Lethal weapon
Michael J. Fox (BttF and Stuart Little).
Patrick Stewart -- X-Men and Star Trek.
George Kennedy (Airport and The Naked Gun).
Christopher Lee for Dracula and . . . Star Wars II, III & Clone Wars?
George Sanders was in the Saint and Falcon movies.
William Powell was in the Thin Man and Philo Vance movies.
Frank Oz for the voice of Yoda in the Star Wars series, and a whole bunch of voices in the Muppets movies
Ann Rutherford was in the "Whistling" movies and the Andy Hardy movies
Mickey rooney in andy hardy and Mickey MacQuire Movies
Sydney Poiter in the Uptown Saturday Night movies and Mr. Tibbs movies
Bruce Campbell has appeared in 3 Evil Dead movies and 3 Spider-Man movies
Michael York (The Three/Four/Return of the Musketeers and the Austin Powers series).
Kane Hodder was in Friday the 13th 7-10 and also in House 1, 2 and 4
Ah, here's a good one that I didn't think of right away: Marc McClure (Superman and Back to the Future). - Edit: IMDb says his BttF 2 scenes were deleted. Wait, really?
Vin Diesel has played Richard Riddick in Pitch Black, Chronicles of Riddick, and voice acted in the straight to DVD animated Chronicles of Riddick: Dark Fury. He was also in 3 of the Fast & Furious movies
Apparently this is a real (short) film: http://www.imdb.com/title/tt0354351/
- Which kinda gives Chevy Chase (Vacation) entrance into the club
(P.S. -- I thought I had posted this earlier today . . . but maybe I just never finished the message. If I did post it and it was deleted by our fearless leader, my apologies for my persistance.)