Well, it was Celmomas II, not necessarily Nau-Cayƻti (who was the first No-God) who we know to be a twin. We don't know if Nau was or was not though. It could be that he was and somehow some process of twinning divorces the soul from a specific body. It could also just be that the process little Kel undergoes (hypnotism of sorts?) does the same. It's totally unclear why makes one "suitable" for No-God operation. But it must be rather uncommon. Just being a twin seems unlikely, given how many people were stuffed in there over the years.
If just being a twin was sufficient, it likely would have works just by blind luck. However, being a twin might be necessary, but not quite sufficient. What the sufficient condition might further be is a subsequent sort of divorce between the soul and (possibly, maybe) the larger ethical realm (read: Spirit, the Soul, the Outside).