Only show the opus number once when there are multiple score blocks

[Robert White]

I have a Lilypond file with two score blocks like the following:

    \version "2.18.2"

    \header {
      title = "Two minuets"
      opus = "Op. 1"
    }

    \score {
      \header {
        piece = "No. 1: Minuet in D major"
      }
      % Music here...
    }

    \score {
      \header {
        piece = "No. 2: Minuet in G major"
      }
      % Music here...
    }

The problem is that the opus number is shown for every score block.
How can I make Lilypond only show it once?

Thanks in advance,
Robert White