Matching the shadows from one image to another?

Yes, if you want to be convincing, you have to use source images taken under matching conditions.  You can flip horizontally, to match light direction, but it is hard to make an image element taken on a bright sunny day, look as if it was taken on a grey overcast day.

You can't just match the shadow only, you have to match the entire image and its perceived color range, which "dumb" procedures such as you propose rarely achieve. You have to read up on how professional color correction is done. Typically you start by matching the midrange colors and only then start crunching or expanding ranges. and of course it can still mean creating elaborate selections and using multiple layer duplicates to attain control ofver individual regions.

Mylenium