Merging 2 photo's 1 side at the center remains unclear

[snowmansion]

   

When trying to scan a centerfold I usually divide them in 2 separate scans which I then try to merge in GIMP.

I open the 2 scans via File - Open as layers in GIMP and then set the canvassize in such a way they both fit. After aligning them I click on Image and then merging all layers together. 

However this issue I then face is that exactly on the middlepoint one part of the image is vague while the other one seems to have a better clear quality.
See the picture attached.  You see that the right part is more yellow and more clear while the left part is a bit vague. 
How can I change this so that all side have more clarity? I now use the Clone/Heal and sometimes the smudge option, however this is time consuming. Is there any way this can be done automatically?

Thanks in advance.