I wanted to see pics. If you find it, even if it's dead, try to take pics of it, or put it in isopropyl alcohol and preserve it until you can take pictures of it. I don't think there are many siamese twin fish out there.
these are def a rarity and dont survive long. i had a friend who used to breed tons of guppys(breed for color and all that jazz) and through all his years of breeding only ever had 1 of these. his were joined at the stomach. only survived 1 week. one swam right side up other swam upside down.
From what little I understanding about fish reproduction, these conjoined specimens are not a genetic or breeding issue, but a fertilization anomaly. It can happen with any quality of stock because it's just a random stroke of eggs not dividing completely.
here they are dead unfortunatly. although they didnt die from this i'll tell you that much, there were a few dead fry. in the thrid pic you can see everydetail. it took me a while to pose them for pics.