RE: Pokémon Center~Sydney (For all of your Pokémon VG needs)
You can only get one of Dialga, Palkia, or Giratina, period, Joey. Trading for another Arceus will not allow you to get another one. HOWEVER you could get another one if you trade over an Arceus caught ingame using the Azure Flute, which has not been released, and probably won't be at this point. But this is the only way to get another 'dragon' as you call it, at all.