I use this tool
http://ngwin.com/picpick to take screenshot. It can take screenshot of "Fixed region" on the screen.
And then repeat last action. Few settings needs to be set in software.
And then GIMP to merge screenshots.
Paste two images to GIMP then set Opacity (in right panel) of second image to ~50% and put pictures together. Set Opacity back to normal, add next image and continue.