[Question] how to specify text subset for multiple fonts?

[smhmd]

WebFontConfig = {
  google: {
    families: ['Droid Sans', 'Droid Serif'],
    text: 'abcdefghijklmnopqrstuvwxyz!'
  }
};

Doing something like this, how to specify that the text subsetting happens only to 'Droid Sans' or to 'Droid Serif'?