I have seen on PS (and, in fact, on a smogon moveset) a Hippowdon with both Slack Off and Whirlwind. However, a Hippowdon can only obtain Whirlwind by breeding with a Shiftry, and can only obtain Slack Off by breeding with a pokemon from the Slakoth or Chimchar evolutionary lines. This means that a set with both moves is impossible.
Incidentally, Hippowdon learns Roar by TM, making this almost completely irrelevant; however, roar cannot phaze through Soundproof, while Whirlwind can. While not the most important of issues, Slack Off+Whirlwind is an illegal moveset.