> each source HTML document starts a new PDF page; there is no way to continue half-blank page with a new doc.

Yes, you are right. There are some tech reasons for that in the current v3.x. If you are sure, there is no style intermixes, try to merge the HTML portions to a single HTML doc before it is passed to PD4ML. Just take into account: a closing or tag stops further HTML parsing, so it makes sense to remove any instances of the closing tags if they are not at the end of the resulting HTML.

Proprietary could help you to prepare a document for duplex printing. The page break tag inserts a blank page if needed to fulfill the “next” condition.