It's an amazing coincedence Sora and Roxas never tried to summon the keyblade at the same time

[Bridgeman]

From everything I've seen and read Roxas used Sora's actual keyblade, which was possible because Sora was using a memory copy in Castle Oblivion.

But still, Roxas existed before Sora even entered Castle Oblivion, and it's a pretty amazing coincedence that both of them never tried summoning it at the same time. What would have happened I wonder?

 

[Blackdrazon]

Roxas may have never actually summoned the Keyblade prior to the opening tutorials, which took 8 days (arguably 9, since you don't have to use the Keyblade in the first tutorial). The games aren't very good at timekeeping, so there's no way to prove it one way or another, but I suppose you could argue that Sora and the others went back to Traverse Town, got back to Hollow Bastion, and cleared out the End of the World in under seven days.

This isn't unreasonable: keep in mind that in order for Xion to be operational by Day 7, Xemnas must have sampled Sora's memories at Hollow Bastion (the fight with the Unknown) prior to Day 7. A KH1:FM player can only fight Xemnas after you revisit Hollow Bastion, so the revisit must have been within 7 days from the first visit. Only the End of the World would have been left to clear out at that point. It could easily fit into however many days remain!

Nomura used to imply that Sora went on several adventures between KH1 and CoM, but they haven't been mentioned in years and probably don't exist any longer. I wouldn't be surprised if Nomura gave up on that idea for exactly this reason: he realized Sora couldn't be using the Keyblade during this period.

 

[Antifa Lockhart]

No, guys. Since Roxas and Sora were two halves of the whole but able to coexist, the Keyblade existed in two places at once. Meaning, Sora and Roxas used the same Keyblade at the same time.