There are 21 actors who have credited roles in films that won back-to-back Best Picture Oscars, but only two of them were nominated for their work in both of those films. Name either one - one got nods for Best Supporting Actress in 1978 and 1979 (winning the latter), while the other scored nominations for Best Actor in 2000 and 2001 (winning for the former).
Meryl Streep (The Deer Hunter & Kramer vs Kramer) OR Russell Crowe (Gladiator & A Beautiful Mind)