What we see here is a so-called Gleichsetzungsnominativ, a form that typically follows the schema of "Something is something".
One of the nominatives is the subject of the sentence, the other(s) is/are the predicative. The rule is:
In case at least one of subject or predicative are in plural, the verb normally is plural as well (There are exceptions to this rule, as always, at the moment I don't see any of them applying here, however).
Note, in your example, the two predicatives are not plural, but we have more than one, so the verb must be plural as well.
An old Duden newsletter goes a bit more into detail and provides some more examples and exceptions.
Comments asked for "how to detect the subject in such a construct": Simply deducing from the word order might end you up with the wrong word - word order is pretty flexible in German.
I seem to remember a rule of thumb: "replace the verb with kann betrachtet werden als". If that doesn't kill the meaning, you got the subject.
Schweigen ist Gold.
Gold kann betrachtet werden als Schweigen doesn't make a lot of sense. Schweigen kann betrachtet werden als Gold sounds better, so "Schweigen" is the subject, "Gold" the predicative.