Back in the real Yen Sid’s tower, everyone is looking anxious and worried about something: whether Lea will do what he is to do. Merlin and the fairies are helping him in a place that is more flexible with time. Maybe Lea will be able to wield the Keyblade (I guess); he has the fire, but just how hot and strong does it burn?
As for Sora and Riku, if Xehanort knows what we are doing, then he needed to have been there when the Destiny Islands were engulfed in the darkness.
It is unlikely he planned that far ahead … unless he jumped across time, according to Goofy, but Xehanort still would need to be in the Destiny Islands, which would be gone. And besides, he’s no good at time travel, according to Yen Sid.
Suddenly, Mickey remembers back to the days of Kingdom Hearts I - back near the end of Sora’s first journey, Xehanort gave up his body and ended up possessing Riku. This greatly shocks Yen Sid, to think that Xehanort could think so far ahead… Mickey wants to go help Sora and Riku.
But how? He can’t enter a dream. Mickey explains that he needs to go to where Riku and Sora will reappear after their test is over. Xehanort will likely be there or coming with him - and we can get him there. Donald and Goofy want to go along, but cannot.
Yen Sid could not find Sora, but he knows that Riku is in the realm between. Mickey goes off to find him, vowing to bring them both back safe!