Since {cairn wanderer} is both a zombie and a skeleton, does {death baron} give him +2/+2?
Nope. Because it's listed as a single ability, it basically says "Creature you control that are Skeletons and/or Zombies get this bonus".
Also, Gatherer rulings -
10/1/2008: A creature that's both a Skeleton and a Zombie will get the bonus only once.
That makes sense, thank you