Hrm. I admit I'm not too well versed in regards of Photoshop macros, but I'm pretty sure there's some way to use variables. Then you could enter the total number of pages and save the left half as
n (with n as a counter), the right one as
total - 1 - n... well, the first page kinda falls off the grid...
Can't think of anything better at the moment. Then again, it might be faster to just do it by hand than thinking / programming a complex macro to do it for you.

...
Or you could try Amazon's Mechanical Turk!
