the easiest way to do this (with the most control)
ok first indeed 2 images will work best (because the light will fall different on the 'object' because of a different spot)
but in any way
1st indeed make sure sharpness blur colour is matched on both images
put the one you want to insert on a layer
then use layer mask
image1
and brush over it (set foreground colour to white, background to black
image2 and when you brush too much, just switch foreground/background colour
in this example
image3 I just made a black background layer and a white foreground layer to show you how I brushed some white in to it (when I have too much just press X to switch foreground/background colour and brush away)
when the result is what you want it to be -> flatten the layers and your'e done...
PS: the position of the feet & the way the light reflects in his neck shows this image was made with 2 shots... no flipping was used...