I don't know of any specific tutorial for what you're looking for, but it sounds like it could be done using several techniques. SPYDERIX has a very comprehensive list of online tutorials -- see faq229-1853
Maybe you could give some idea of what level of expertise you have, or what source images you're working with. For example, are you working with people's profiles or is one person going to obscure the other person? You may need to read up on compositing techniques (layer masks, etc) and if you have CS, there are some useful tools for balancing the color between two images.
Without knowing more about your source images, I would say bring one in as a new layer on top of the other image. Use a layer mask to isolate it, then free transform to match the sizes. Try that to get you started, and feel free to come back with specific questions.