Adjustment of two birds

I have two parakeets, Bella and Kiwi. I got Kiwi first and she has always had a bad temperament. I thought it might help to get her a friend since I am at school all day. Hence... Bella, soooo sweet. I separated them for a few days when I got them like I was told to do. But Kiwi still has not gotten use to her and it's been a coule months. She is frequently pecking at Bella and running her away from one of their food dishes, treats or toys. Today I noticed Bella's droppings are a little runny. I'm worried that she may not be getting enough food. What can I do to have my birds adjust to one another?