Hi deadants,
To add to your excellent explanation, it is worth noting that this method can also be used to add in symmetrical bits that have been severely damaged or lost in dark shadow, such as one side of a face or an eye, or an ear etc (or even one side of a symmetrical object)
I usually add a "New Layer" to the second image, and then "Paste" the saved selection from the first image into it, and then move it to the required place.
To get the reversed image (for symmetry) use: > image > rotate > Flip Horizontal ( for left / right symmetry) {or > Flip Vertical (for up / down symmetry)}
Once positioned as required the Eraser Tool can be use to remove surplus bits in the selection, and the two layers merged using: > Layers > Flatten image
Any difference in background colours can then be sorted out using the Clone Tool
Hope you find this useful
Mudge