sora(and roxas) could only dual weild after defeating xion, I think xion spliting sora and roxas's memorys also split the keyblade, alowing dual use, even when xion returned to roxas the keyblade was still split like a scarr or somthing. and as for why sora had a keyblade without right of succetion"ik bad spelling" think of this, why does ven, aqua, terra,mickey, eriques, and old man xehanort have keyblades? cuz there hearts were strong thats why. sora's keyblade showed up when destiny islands was threatened light shines greatest in the dark. riku didnt get 1 cuz at first his heart was in confusion over terra and xehanorts influences, and only took soras cuz he started settling torward darkness round that time. kairi didnt get one till riku gave her it "most firetrucked up thing in kh, sexism ftw =.=" phew theres my theory. btw riku x shiki ftw!