The duo will appear in the Season 4 finale and are expected to return for Season 5. "Glee" has yet to officially be given a fifth season, but a renewal is likely.
Duke recently appeared in episodes of "Drop Dead Diva" and "Hawaii Five-0." She's primarily known to TV audiences thanks to her 1960s series "The Patty Duke Show" (see the video below for a refresher). Duke won an Oscar for her portrayal of Helen Keller in "The Miracle Worker" in 1962. She has two Golden Globes and multiple Emmys to her name, including one for a TV remake of "The Miracle Worker" from 1979 where she played Anne Sullivan.
Baxter, known for her role as Elyse Keaton on "Family Ties," made headlines when she came out as a lesbian in 2009.
For more spoilery details about how Duke and Baxter will factor into "Glee," click over to E! Online.