Aggresive Behavior

I have had a male cockatiel, Loki, for over a year. He is not the friendliest bird and wont come out of his cage most of the time. Since he doesn't like being handled as much, I found a female cockatiel, Freya, for him to spend time with/bond with (hopefully). I had my parents watching them while I was visiting a relative, and they were put in the same cage. They got along fine the first day, Loki was thrilled to have a friend and Freya was very comfortable as well. But after the first day Loki started picking at Freya. I don't know what he is doing, as it looks aggressive and he has been seeming to act territorial. I moved Freya to a different cage, but they didn't want to be apart. Loki still tries to pick on Freya but they both refuse to go in separate cages. Could rearranging the perches and toys help with Loki's aggressive behavior?