Here's why:
* Individually, they are singular: "cup" and "saucer" are both singular nouns.
* Combined, they act as one: When used together, they describe a set. You wouldn't say "I have two cups and saucers." You would say "I have two cup and saucer sets."
Therefore, while it's made up of two singular nouns, "cup and saucer" as a whole functions as a singular noun.