I faced a very similar problem after scanning a book over our university's scanner system. I didn't find a suitable software for that purpose, so I simply wrote one myself
It's basically a Linux bash script that extracts the two pages from the pdf, does some image manipulation like contrast and gamma correction (using ImageMagick) and writes back a PDF file with the optimized version.
If you're interested and have a Linux box around I will upload an pre-alpha version of it...