First of all is it actual biting? or more light nibbles? Chins will nibble on you to show affection, basically trying to groom you. If it's actual biting then how old are they, I think you said 6 months in another post? At around 4-6 months they do go through puberty, which lasts a few months. Basically think bratty moody teenager, they generally out growing it, but are testing to see what they can get away with. Personality changes are not uncommon during puberty either, and they don't always go back to being the same as when they were kits.
You say you have to hand feed since the other hogs all the pellets? If you have two chins in one cage you need two food bowls and two water bottles, and in some cases two hay holders too to prevent resource guarding. If one is preventing the other from getting food or water that can lead to a fight, and chins can and often do fight to the death. The way I have it set up for my two is I have one food bowl and one water bottle at the top and bottom of the cage to prevent one from being able to guard all food. I also put hay in 2 different holders, and put hay cubes around the cage. Hand feeding pellets can be used for bonding, but you shouldn't have to do it just to make sure the chin is getting enough to eat.