Edward G. Robinson in **The Prize ** (1973) played both the good, Nobel-prize-winning scientist and the evil, Commie spy scientist who was trying to replace him.
Brent Spiner playing both Data and Lore on ST:TNG.
I would watch any sort of Alton Brown/Swedish Chef crossover. I can completely imagine Alton pulling that off.
I’d also love seeing a Taco the Octopus from Deep Fried ™ on Good Eats.
More cheese (and this thread is beginning to smell like a pizza): Soaps have done the good twin/bad twin meme to death. Off the top of my head, there’s Kitty & Kelly on All My Children, Tony & Andre on Days of Our Lives, and Marly & Victoria on Another World.
Barbara Eden played both Jeannie and her nemesis Jeannie 2 (the anti-Jeannie…brown hair/green clothes).