If you are going to get two birds at two different times you would definitely need two cages. You might be able to put them in a cage together at some point and time but to be able to quarantine them (are they from the same batch) and work with them they would need to start out being separated.
If the two birds are already housed together then you can of course house them together from the start when you get them. It is harder to tame birds if they already have a companion bird in with them. It is not impossible just harder. If the birds are from the same batch and turn out to be one male and one female you may also need to separate them. If both are male or both are female it "should" be okay. Some people have problems with one sex fighting more than the other. We have two male housed together that get along fine and a friend has two females that get along fine so I think it depends on the bird as far as their attitude.
When you start taming them it will be easier to work with them if you can take one at a time into a room away from the cage and their companion. In this way they will not feel as strong of a need to get back to their cage and their buddy and should pay attention to you a bit better. You will probably have some problems with chirping or screaming when you separate them to work with them but it is a lot harder to work with two untrained birds at the same time who see each other as a companion and you as a stranger.
I am all for a bird having a companion but I usually suggest getting one bird housing it in its own cage and working with it until it is tame. Then if you must have a companion for it get another bird in a separate cage and work with it until tame. By then quarantine should be over and they will have gotten used to hearing each other in separate parts of the house. Then you can move their cages closer and introduce them in neutral territory. If that goes well you can put them in a "changed" cage one of their previous cages all changed around or in a completely new cage.
If you do not have room for two cages I would not suggest having two birds. If at a later date the birds start fighting or one becomes sick, injured, etc. you will need to separate them. To do this you would of course have to have a second cage.