[ul][li]Henry Fonda in Once Upon a Time in the West and Yours, Mine and Ours[/li][li]Wallace Shawn in Vanya on 42nd Street and Clueless[/li][li]Michelle Pfeiffer in Scarface and Grease 2[/li][li]Colleen Dewhurst in Annie Hall and Ice Castles[/li][li]Julianne Moore in Far From Heaven and Evolution[/ul][/li]
They came out a year or so apart, but are worlds apart in terms of themes and quality. Others?
Brendan Fraser in George of the Jungle and Gods and Monsters
Johnny Depp in Once Upon a Time in Mexico and Finding Neverland
Robin Williams in Popeye and Insomnia
(many possibilities there)
Ed Wynn in Requiem for a Heavyweight and Babes in Toyland
Tony Curtis in Some Like it Hot and The Boston Strangler.
Jack Lemmon in Some Like it Hot and Glengarry Glen Ross.
Raul Julia in Street Fighter and The Burning Season.
Jamie Foxx in Ray and Stealth.
You can, of course, come up with this for just about any well-known actor: most made both bad and good films (and films that were wildly different in theme) through their careers.
Raoul Julia in Street Fighter and Kiss of the Spider Woman.
Humphrey Bogart in The Return of Dr. X and The Roaring Twenties