This is true only if both are in the same layer. Normally the watermark is in the header/footer layer which will be behind the document layer.
Watermarks in Ribbon Versions of Word

If both images are in the same layer you can order them.

I might recommend copying the watermark from the header and pasting into the page itself as an image. This will be far from perfect but may work. You would want it to essentially overlay the watermark that is in the header.