I've done a lot of this exact same thing you're trying to do.
Before affordable scanners, I made scaled transparencies and did overlays. I have a darkroom, but you could do this on a photocopier, using the reduction/enlargement settings to make the scales agree. I also used overhead projectors and slide projectors to project my transparencies onto a large map taped to the wall. That way, you can scale on the fly by moving the projector closer or farther away.
At home, you could print them on the same sheet of paper, or one of them on transparency sheets and use a light table to light them from behind. A 35mm with double-exposure capability could be another method that might work. Or you could shoot two separate slides and sandwich them in a single slide frame. I think my Nikon D200 allows multiple exposures on the same image, but I've never played with it.
A cartography buff would hand-draw the details.
Still, the easiest and most flexible way is Photoshop or a clone that allows layers with adjustable transparency. You can scale till it's just right and adjust how much one image overlays the other or even change the order of which layer is on top. Then "freeze" the result as a new file, keeping the originals intact.
Modern digital camcorders have fade and dissolve options that might yield some individual frames that merge your two maps.
There's so many ways to achive similar results, I probably left a few out!
Good luck, the results can be very revealing!
-Ed