(Again, sorry for awful english)
I have XML, where titles are in several lines:

...
<title>
  <p>Chapter 1</p>
  <p>Introduction</p>
</title>

but the PDF bookmarks are shown as "Chapter 1Introdiction". Is there a way to define something like "divider"? I want bookmark to be "Chapter 1 Introdiction" (or similar).

This should work, as it should be picking up the whitespace between the <p> elements, unless it looks like <p>Chapter 1</p><p>Introduction</p>?

Yes, my mistake; the original XML is exactly as you shown. Is there a other way to fix that (by CSS)?

At the moment I'm not sure if there is, as the prince-bookmark-label property will take the textual content of the element by default, and there is no way to say take the text from the first child, then add a space, then the text from the second child. You can specify your own text, like "Chapter 1 Introduction", but that will involve duplicating the text from the document.