First, I must ask you why you want to breed and if you understand the many problems that will arise.

Do you know what eggbinding is and how you can prevent it?
Do you understand that many many budgerigars are prone to egg cracking/chick killing? How will you cope with this? My two breeders killed 10 chicks, and it was only their second clutch.
How many clutches do you plan to have?
How old are your birds?
How many nest boxes are in the cage right now?
How big is the cage? Breeders should fly for that extra breast muscle mass. Strong muscles are said to help prevent egg binding.
What diet are your birds on now?
Do they receive calcium supplementation? Breeding hens require abnormal amounts of calcium.
What will you do if the hen abandons the nest?
It is very likely your two hens will attack each other for the right of the nestbox. Again, the size of the cage plays into this.
What will you do with the chicks after they are weaned?
How do you plan to stop the hen from laying multiple clutches?
How will you prevent splayed legs?
Lastly, WHY do you want to breed?

I ask this from an experienced point of view. I remain in contact with many of my local breeders and I converse with them frequently. My breeder budgerigars have also produced two clutches, one of which went fantastically and the other which was pure terror. If you cannot answer the questions above, I guarantee you something might/will go wrong. I hope you reply soon