Sounds like Schroeder doesn't take well to tankmates. Bettas are like cats and all have their own little personalities. Some will take better to company than others, you'll probably have to separate them.
yeh i would seperate them, its not fair on the coreys. Whether Schroeder's doing it for fun or no, & with the signs of nips on them already, you can't be certain he won't do another that could kill them.