This might not be the best way but this is how I'd do it.
Open them both in photoshop.
Create another blank image a bit over twice the height and a little wider, just to give you room to move the images around.
These images were 800 by 582 so I went 900 by 1300.
Then paste both original pictures into the new image on different layers.
Give which ever layer that's on top about 60% obacity so you can see both.
Then using the arrow keys to move pixel by pixel line up a common point on both pictures.
I went for the curve of the C in Cheese.
Once they overlay exactly, return obacity to normal on the top layer and they should look like one continuous image.
Except in this case there is a bit a under shadow on the top image probably from not being completely flat when scaned.
So then I grab the top image, cut out the bit on the bottem from just under the smaller swirl on the left.
This helps the image match better but not perfect.
So on the top image use the eraser tool with a soft round 40 pixel brush on it and work along the line deleting edge so it not so harsh and it should blend them togther.
It helps that the image is painted so the colour is irregular anyway. With something computer coloured you would probably have to spend a lot longer on the blending.
Then just merge your layers and use the rectangular marque tool to grab it as close to the edge of the green as possible and copy and paste this into new file.
Once you hit the copy keys and then go to create a file, it automatically makes it the size of your copy.
I just add the last bit because of the slight irregularities you can end up with the angle of the page between both scans.
Really hard to avoid this without a fancy pants scanner. These pics where very close bit still a little off.
Hope this helps.
I wasn't sure how well you knew photoshop, so I think my description varies between being too vague and too detailed at different points.
Let me know if you need anymore help.