Problem with assembling flattened PDFs with XFA docs



I have a flattened PDF (legacy) doc that I am sending into a livecycle process to merge with an XFA doc that I am creating on the server. Assembler will not allow me to assemble the docs in the following order:

1. flattened PDF

2. XFA doc

I CAN do the reverse order just fine. Problem is order DOES matter and I need the XFA last.

Any thoughts on how I can reach that end result? I was thinking of doing the merge in the wrong order and then maybe reordering the pages, but don't know how to do that either.

Any help would be VERY appreciated!