No, I'm not talking about the Teen Titans. I'm talking about something from Kingdom Hearts.

 

So, in KH 3D, Roxas reminds Axel that he made to him a "promise to bring them back", & in KH 358/2 Days, Axel reminds Xion of the same thing. I just can't help but wonder whether or not if they're talking about the same thing.

well i got the impression they were

Yes, they were talking about the same thing. First Axel said this to Xion referring to both friends, then when Xion disappeared it was said she was "absorbed" by Roxas thus gaining her abilities and memories (sora´s memories) also being the reason Roxas can double-wield the keyblade. Once Roxas delivered everything to Sora he disappeared. Now to the point in KHDDD, we see this scene with Axel and Roxas where he tells him "you made us a promise, that you'll always be there, to bring us back. Got it memorized?" this was inside Axel's/Lea's mind as he was asleep once returning to his human self. It was a reminder for himself about his motivation and about what his objective was, to bring his friends back. Once awake he started his quest in order to fulfill this task.

 

So yeah, it is the same thing.