Hi John - I think you might find this link provides the answers you need. I seem to remember that a golden rule in this sort of work was to ensure that the two images have their light sources coming from similar directions. Also I think you have to work on the colours and match them before you combine them - only ever done this once for a laugh in PS7 so that should give an indication of how long ago !! Anyway this should answer your question.
http://en.allexperts.com/q/Adobe-Pho...hotographs.htm