I like this idea because we'd finally learn more about Marluxia and Larxene. But it is hard to imagine them helping Sora.
Maybe they could trick Sora into thinking that they want to be redeemed by helping him fight against Xemnas but secretly plan to kill Sora when the timing is right or something like that?
Sora may be gullible, but I doubt he'd trust 'em regardless of whether or not he could remember Castle Oblivion. I'd rather have 'em both return as villains in the next arc and learn more about 'em there~