Perhaps they need to eat a bit more. Are your worms large? My worms are quite little so my five month olds can easily eat three each day. One of mine is really large, she (I think) is 21cm now. Juveniles seem to need a lot of energy due to their growth spurt. If yours are adults feeding every two days is OK, however they could probably eat a couple of worms each. Unless your worms are very large. They probably were hungry after four days, one must have mistaken the other for food. That's great that you have a division now, that will help your injured one recover. You will be amazed at how quickly they recover. I was away over Christmas and came back to a bleeding axie, he/she was missing one back foot and one back leg. They have grown back perfectly now and I can't quite work out which one was injured. So they do have amazing regenerative abilities, however the gills don't seem to regenerate like the limbs.