combining scans

[Espermaschine]

I have scanned an old recordcover with my DIN-A4 scanner.
The cover is bigger than the scanner so i had to scan it in two parts.

Now i try to make a composite of it, but the images are not exactly the same colour/value.
I tried eyeing it with the Curves or Levels Tool but i cant get it right.

Is it just a matter of value, or do i need to adjust all the RGB channels ?
My guess is, the differences come form the scanners light, so that seems to tell me its just the value that needs tweaking.


I also tried the method with sample points, but i can never get it right....

Any advice, or is there a way to automate this ?

EDIT:
added a link to the xcf

http://www.mediafire.com/file/v18vvsmy5p...can.xcf.gz