When doing widow and orphan control, our designer normally ensures that facing pages have the same length: For example, if the right-hand page was shortened by a line to avoid a widow on the next page, then the left-hand page also gets shortened by a line so that the facing pages have the same length.

Is this possible with PrinceXML?

No, not yet.

Is it something you are looking into in the future?

I thought I could live without it, but it turns out to stand out much more than I would have thought once I am holding a book in my hands.

We do this manually, adjusting word- and letter-spacing on individual paragraphs, exactly as we would using Quark or InDesign.