I posted a batch dekeystoning process using Photoshop which can create two pages of output for each page of input here
. Steve's Book Scan Wizard does most of the same things, and more, but if you already have Photoshop (version 6 or higher) and don't want to duplicate your input into L and R directories, it may be worth a try. Someone in that thread mentioned another alternative to Photoshop, which I haven't tried myself.
The tutorial itself deals with separate L and R images, but I've tested it with a single 2-page input like you're proposing, and it handles that just as well. Anyone who's actually worked through the tutorial there should be able to create the 1-in/2-out script, but if you have questions just ask.