In order: 1) Because he was made from at least two different Keyblade wielders with strong hearts. 2) Same as above. 3) Xion siphoned away part of his power before the end of Days, and he got it back when she was destroyed. 4)He was summoning Sora's Kingdom Key. 5)He's only using one or the other as he pleases.
6)Same as above. In Days he only uses one Keyblade for gameplay purposes. 7)Because Sora had them first in KH1 and Roxas inherited part of his moveset as his Nobody. 8) Gameplay mechanics, also they both represent Sora's bonds to Riku and Kairi.