Did that almost reasonable 24-month turnaround time between Final Fantasy XIII and the just-released-in-Japan Final Fantasy XIII-2 rock your understanding of reality? Who could blame you? The development time of the last "direct" sequel was also pretty short -- two years between Final Fantasy X and X-2 -- but it was five years between Final Fantasy XII and XIII. Did Square-Enix make some dark sacrifice to summon the new game so quickly?
You'll probably need to talk with Tri-Ace about that. According to Tri-Ace programmer Yuichiro Kitao (courtesy of Twitter), the Star Ocean/Valkyrie Profiledeveloper assisted with design, programming, and art on the all-too-rare Final Fantasy sequel. Animal sacrifice may have also been involved, but that's not the kind of thing you talk about on Twitter. That's okay, Square-Enix; even Santa had the elves for help with that Christmas(ish) miracle.