My Perfect Mistake but also an excellent professional photo artist, is answering this question in his astonishing "Genotype" photo series.
Photographing two relatives in exactly the same light and surrounding definitely is exciting (and difficult) by itself. But taking one half of each face and putting it together as one gives you the real mind-blowing effect... but see for yourself! These are examples of twins, mother and sun, father and son, mother and daughter and sister and sister:
|Mother & Son|
|Father and Son|
|Mother & Daughter|
You can check out the whole series on www.cherryinger.com/portfolio/genotype and you can also order the book with the whole series on this website. Enjoy and thumbs up for this rocking photographic idea!!