It's old software but I believe this can also stitch 'flat' panorama's:
See "Stitching flat scanned images": http://hugin.sourceforge.net/tutorials/scans/en.shtml
See "Stitching flat scanned images": http://hugin.sourceforge.net/tutorials/scans/en.shtml
Stitching flatbed scans is not the best use case for Hugin and it shows.
I used the same tutorial linked by Op http://hugin.sourceforge.net/tutorials/scans/en.shtml
Note that Hugin requires overlap between scans. I would test MS ICE first if I were you (less powerful, but way simpler) https://www.microsoft.com/en-us/research/product/computation...