If the compilation was available as a compilation on a prior art date, then it is a single prior art reference because its a publication as a compilation.
If it was separate articles that are prior art, and then compiled later down the road after the invention by applicant, then it'd be separate pieces of prior art.
But remember, a single reference doesn't automatically equate to anticipation. In fact, while a compilation is a single publication, it inherently discloses multiple embodiments (each part of the compilation) and therefore whether a single embodiment anticipates or multiple embodiments render obvious is an entirely different question.
So for those conference proceedings compilations, if you're using two articles, you'll need to make a single-reference obviousness argument.