You can use any s/w that allows you to mask off an area of one layer showing the second through. If your masking allows feathering of the mask then all the better.

Short of functionality called masking you could achieve the same thing with a 'crop' function.

This is not a difficult effect to do but it can be quite hard to get 'just right'. The trick is to make sure the lighting is exactly the same on both takes or it will be noticable. Hence, only do this using artificial (i.e. controlled) lighting and not natural light.

__________________
I'm not young enough to know everything!