That is quite unusual, both cases. The one in the picture would have been interesting, both appeared to be growing well, just with a joined tail. In human Siamese twins there is always a dominant member of the pair. That's the case with the aro in the video. Ok it wouldn't have survived long in the wild. But perhaps the smaller "parasite" enjoys being dragged around. I think it's quite interesting how many creatures adapt to the conditions they find themselves in.