do you have an example?
 

If you use Photoshop, you can use the picture with the mirror as the background copy. Cut out the picture to put on the mirror. Drag it to the background and resize with the edit / free transform tool. I would then change the opacity to show a little bit of the original mirror showing through. It sounds simple, but if you are not familiar with photoshop it may sound like a different language.

Hope this helps.
 

I have used this technique with my husband and son! I set my camera up on a tripod in a position that would remain that way for both photos! I had my husband stand in front of the sink shaving and then I had my son do the same making sure I never moved the camera! I made sure I was able to capture both the reflection and the real image in both photos! I then cropped my husbands reflection from his photo and overlayed it onto my sons reflection! I call it Man in the mirror! Yours is an awesome idea! I love it and would love to see the finished product! ;D You can see the image I am referring to in my ablum called reflections! I have it in Sepia and color! Have fun with it!
 

I would LOVE to see this!! It sounds like an awesome picture!!!
 

I too would like to know how to do this. Does anyone know how it can be done on PSP?
 

As suggested above, you would have to be sure your two images are from the same set/camera position. That'll make editing easier and it will look better.

You can use the lasso tool on PSP and cut the mirror image out and paste it over the mirror on the 2nd image. It all depends on what the image that you're working with looks like. There may be some tweaking that would need to be done in different photos.

Good luck!