1) Buy 2 seperate 800 codes (if you plan on swapping to a main. 1 800 will do fine to a fresh silver)
2) Swap the gamertag to something random
3) Go to the main account and change the gamertag to the one you just swapped.
If it's a fresh silver:
1) Get a turboer (or 5 lol)
2) Make a silver account and input the account credentials. Enter the gamertag you will be swapping.
3) Press start and swap the gamertag. Your new fresh silver will have the tag the moment it becomes available.