(Help!) My Tiel is a big bully :(

I have a tame male cockatiel and I recently got a female. They got along at first with little problem, so I kept them in the big cage together. The female has become territorial and is now bullying the male who is non conformational. For example, she hogs food and certain parts of the cage and will make him move if he tries to go near her 'spot'. She hisses and even tries to bite him sometimes. I separated them a few minutes ago after she tried to nip him. What should I do? Is it normal for female tiels to be more aggressive?