Another double copy, this time already written about in this post. I cannot add anything more to that one. Going though my collection makes me aware of the many records that I can and should take out. Some are not good enough, others are like this: double.
In the nineteenth century the piano works for four hands were considered very intimate. Two players sitting next to each other, who knows what might happen? Sometimes hands might touch, so there certainly was some erotic tension. Many composers were also hired to teach (usually quite young) pupils, only to fall hopelessly in love. The juicy results of these power dynamics are not only of the last decades, so much is clear. Of course these works can also be performed by siblings, so there are a lot of twins, identical or not, and from both sexes. At a delicate age they considered cute, as they get older they are getting weirder (take for instance the Labeque sisters).