New cat kicks old cat out of home

Does anyone have any suggestions on how I can get two cats to accept each other. Unfortunately some damage has been done. Our Manx cross Bella only comes home to eat at nights now as our latest addition attacks her and chases her away. We keep them separate when Bella is inside but she just takes off as soon as she has finished eating. We have to find a way for Tasha to accept Bella and leave her alone. Any ideas? I don't want to have to find new homes for either of them.