Google has updated the structured data policies page, specifically the section on having multiple items on a page with structured data. It added whether you nest the items or specify each item individually.
The updated can be found over here.
Here is what it looked like before (click to enlarge):
Here is what it looks like now (click to enlarge):
The new version gives clearer examples, show screen shots of what it is describing, it explains the difference between nesting and individual items and then gives specific code examples.
Google added “Note: If there are items that are more helpful when they are linked together (for example, a recipe and a video), use @id in both the recipe and the video items to specify that the video is about the recipe on the page. If you didn’t link the items together, Google Search may not know that it can show the video as a Recipe rich result.”
Forum discussion at Twitter.