Ok, this is not really a theory and more of a discussion to work things out. Some of this is based on things stated here in this video, which of course, I highly recommend. He's always quite detailed.
https://www.youtube.com/watch?v=7swFXDbxegw
But here is the 'theory':
Terra and Ven are not physically in the Realm of Darkness. They are just projects, but it is 'them'. Think of how Mickey appeared to Riku while he was in Castle Oblivion, or the unexplained occurrence of Terra and Ven's keyblade coming to save Aqua at the end of Birth by Sleep.
Here is the sketchy part: I think this is vaguely related to the script Nomura had Terra and Namine's VA act out during the concert, as Dean mentions in the video. Namine insists that Terra will be touched by a familiar light, and that he must guide Aqua so that she isn't confused by darkness. When he says he doesn't know how, Namine tells him to stop whining and figure it out. (not actually)
I kind of consider this event canon... but in the context of this theory, it could be just semi-canon. A metaphorical concept that Namine was able to stir Terra enough to project himself into the Realm of Darkness and that she possibly did the same for Ven. Timeline wise, it doesn't make a lick of sense. 0.2 clearly ends when the worlds are restored thanks to Sora. That is before Namine loses Sora's memories and starts to restore them in the first place. It's inconceivable that she'd be talking to a Terra of the past to ask that he help Aqua... in the past.
But if we take it as only metaphorical, that something guided Terra and Ven to go help Aqua out... I'm willing to accept that I guess.
Back on point, those 2 figures are Terra and Ven reaching out to help Aqua just like their keyblades. Terra is obviously still fighting his literal inner demon since Terranort shows up to crash the party. Ven cannot communicate properly and can lead her silently, like he's sleepwalking. (just like he lead her to the destroyed Land of Departure)
https://www.youtube.com/watch?v=7swFXDbxegw
But here is the 'theory':
Terra and Ven are not physically in the Realm of Darkness. They are just projects, but it is 'them'. Think of how Mickey appeared to Riku while he was in Castle Oblivion, or the unexplained occurrence of Terra and Ven's keyblade coming to save Aqua at the end of Birth by Sleep.
Here is the sketchy part: I think this is vaguely related to the script Nomura had Terra and Namine's VA act out during the concert, as Dean mentions in the video. Namine insists that Terra will be touched by a familiar light, and that he must guide Aqua so that she isn't confused by darkness. When he says he doesn't know how, Namine tells him to stop whining and figure it out. (not actually)
I kind of consider this event canon... but in the context of this theory, it could be just semi-canon. A metaphorical concept that Namine was able to stir Terra enough to project himself into the Realm of Darkness and that she possibly did the same for Ven. Timeline wise, it doesn't make a lick of sense. 0.2 clearly ends when the worlds are restored thanks to Sora. That is before Namine loses Sora's memories and starts to restore them in the first place. It's inconceivable that she'd be talking to a Terra of the past to ask that he help Aqua... in the past.
But if we take it as only metaphorical, that something guided Terra and Ven to go help Aqua out... I'm willing to accept that I guess.
Back on point, those 2 figures are Terra and Ven reaching out to help Aqua just like their keyblades. Terra is obviously still fighting his literal inner demon since Terranort shows up to crash the party. Ven cannot communicate properly and can lead her silently, like he's sleepwalking. (just like he lead her to the destroyed Land of Departure)