This example from the css-tricks article about <use> has an <svg> with <use> tags that reference a node from a different <svg> on the same HTML page.

Where do the specs state whether this is allowed or not?

The specs link to RFC 3987 for the definition of "URL reference". And they define <use>'s href attribute as "An URL reference to the element/fragment within an SVG document". So we have two "SVG documents" inside an HTML document? Where does RFC 3987 encompass this situation?