Thanks for the helpful posts Zoddy.
Back to the original subject now please. I have seen quite a few conjoined fish over the years but they seldom survive very long. Almost always there is something missing in their systems that ends up meaning they starve to death in short order. Once in a great while one will survive long enough to start looking like an adult fish, with an appendage. I have yet to see two fully developed fish that were joined together.
These fish were joined at the anal fin. The second day they weren't evident, and I'm assuming they were so lightly conjoined that they pulled apart. I only wish I could have gotten a photo of them first.