My one lovebird attackes my other lovebird?

i have 2 lovebirds which i brought 2 months ago one is yellow colour and other other one is green the green one always bites the yellow one and does not allow it to sit on the pirch when green one is sitting on it only during the night time they sit together and sometimes rgey sit together to eat and to groom each other but that happens rarely how can i help them to stay in peace with each other and since last week the yellow ones feather is also following is this the sign of any ilness the feather falls when it grooms itself