Dunno about that theo guy but Ghouls and Humans United because they had a common enemy to defeat. Common trope. Meliodas and NNT didn't had that chance at all. Not a third party stronger than both Demons and Goddesses in order for them to unite and cooperate. Code geass? Lelouch literally did what Mel did by betraying his old comrades in order to achieve the zero requiem for the greater good. Same logic for Mel, he wanted to work toward a plan for the greater good but he didn't had the same chance as Lelouch because both the SD and the DK surprisingly allied themselves in order to literally kill him lol.