To install the tool, extract the zip and save in the Javascripts folder. You may need to create a folder named 'Javascripts' if there isn't one there already:
It will add a menu item "Combine Even Odd Pages..." under Organize ribbon (ribbon UI) or the Documents menu (classic UI). To use, first open both documents -- they must be saved first -- then run the script: Select which document is to be used for odd pages, which is even, and if either of them need the page order reversed. You can pick whether to insert the pages into one of the above documents, or create a new unsaved document that combines the files.
Limitations:
- Files must be saved first. doc.insertPages() cannot access files that are not saved.
- If one of the documents to be combined is longer, it puts the additional pages from the longer document at the end
- May be rather slow on large documents
- It doesn't handle the bookmarks: If you combine to a new document, then there will be no bookmarks. If you insert the pages into a document, it won't import the bookmarks from the other document, or change bookmark order if you change the page order.
Maybe this is already a part of PDF-XChange and I've just not noticed - if so, let me know!