Visual Search Documentation

Looking at the Visual Search documentation I see there's a filter for URL selection:

The KnowledgeRequest object includes the following fields:

• filters — A Filter object that you use to filter the results.
  • site — The site filter returns similar images and similar products results from the specified site only.

documentation here

Is there a way to format this site filter key with multiple URLs? If I would like to search multiple sites for a visual search match, do I need to make multiple queries?