A tilelive source that merges sources.

Overzooming (using part of a lower-zoom tile), masking (falling back to lower-zoom tiles when they're intentionally missing, as in the middle of the ocean), and image layers are all supported and the most conservative upstream values for Last-Modified and Cache-Control are used.

merge:?source=mapbox:///mapbox.mapbox-streets-v5&source=tilejson%2Bhttp%3A%2F%2Ftile.stamen.com%2Ftoner-labels%2Findex.json

The full-on explicit way (avoids having to encode URIs with encodeURIComponent in some circumstances--this is the same as url.parse(uri, true)):

{
  protocol: "merge:",
  query: {
    sources: [
      "mapbox:///mapbox.mapbox-streets-v5",
      "tilejson+http://tile.stamen.com/toner-labels/index.json"
    ]
  }
}

(Yes, both source and sources work.)