As the others have mentioned, you need to separate and treat.
As to why they are pecking, the brooder may be too hot or it may be too crowded. They are getting bigger and may need more room. Do you use a red light or a white light? A red light keeps them much calmer.
Their feathers are growing in and may have blood in them. They may be pecking at the red shafts of the feathers. The red light will help if this is the case.
Watch and see if it is one specific chick starting the pecking. After the other chick starts to bleed, they will all join in, but often one chick will start it. If that is the case, you can try separating the perpetrator for a while.
Chicks are social animals. They really get lonely and will let you know it with their chirping. It can be loud as they are trying to get the attention of the others. If you can, when you separate them keep them in site of each other. You can put a mirror in with a lonely chick so its reflection can keep it company. A feather duster or small stuffed animal (as long as it cannot pull the fur off to eat it) will also work.
Blood has been drawn, so you do need to act. It can be frustrating to try to find out what the problem really is. Good luck!
Just a few suggestions. The red light is probably your best bet.
