Difficult question to answer as it depends on the type of bird, their ages and how they are cared for. If you have them together in one cage you need to make sure one is not picking on the other. Most birds are territorial and do not get along with another bird in the same cage unless they were raised together. Keep an eye out on their behavior when you're around and the condition of the birds when you're not around. If you see one bird is not looking too well, i.e. ruffled feathers, missing feathers, change in behavior, not eating, etc. you need to separate them. On the other hand, you may be lucky and the birds are happy be companions.