Excellent information Alexandra
It is definitely sticky worthy
I would like to add...
When it is time to place your budgies together in the same cage, I do encourage people to re-do the entire cage before adding in the budgies. By rearranging toys, perches, food bowls, and other objects in the cage, there will be no "marked territory." The rearrangement makes it so that the cage is neutral, rather than "owned" by the budgie that had been previously placed in the cage.
Also, when it comes to training your budgies together, it is ideal to train them separately, but it is also a good idea to train them together. Like Alexandra noted, when one budgie starts to warm up to you, the other will usually follow along. Budgies are generally, "monkey see, monkey do." By training them together, you're allowing yourself to be one of their flock. It is great to have one best friend, but it's even better to have a group of friends too, right?