There are only two was to merge two halves of an image together (I assume that's what the OP is asking). The easy way or the hard way!
Doing it the easy way where you just rotate and line up the two halves of an image then any good free programs like IrfanVeiw is recommended, Photoscope just appears to be a clone with less functionality. GIMP is also good.
However the easy way usually produces crap results, even when using Photoshop's auto join tool, because too many people just slap the two halves together and call it a "join", "stitch", or "merge" without bothering to edit out the join. To join two halves together properly and make it look both good and seemless you need the time, the tools, and
a bit of skill. That is the hard way.
You can edit out the join using free programs, but for the hard way you ideally need something that supports layers because having a layers tool is a massive boon if you want to do a good job of joining. Photoshop Elements does the job. Whilst not as good as the expensive full blown Photoshop, it has enough tools to get you by and most importantly it supports layers!
Example of the hard way done in Photoshop:
1) Images to be joined together. My own scans, I always scan a centre section to aid with lining up and later repairing.
2) All three images lined up together. This would be the end of the easy way.
3) Time spent repairing the image to create a joined/merged image. Note: this is only showing the layers of the repair work done.
4) With all the layers switched on we now have what looks like one whole image without the spine gap or join lines. Image is later flattened and edited further for colour correction, etc; and yes the original mag pages really did have that bad of a red cast.