On photoshop, you first scan both halves and save them as individual files. Then just go file/automate/photostitch.
Then, you select interactive placement, select the two halves and hit OK.
It will open both files and try and place them like they should. You just move them around to their proper position.
It helps to have the images roughly aligned and in the proper orientation first.
I've begun drawing a custom film holder for such a task on my 4490. Will need to get working on it one of these days, especially since I've got 90% of the plan done. I was thinking of getting it laser cut but the overly expensive service made me go back to cardboard and an xacto knife... and possibly Popsicle sticks.