It certainly seems that Nichiren himself felt that there was exactly one correct doctrine and one correct practice. Example:Coëmgenu wrote: ↑Sat Feb 03, 2018 10:33 pmOrthodoxy & orthopraxy seem united in Nichiren Buddhism, like essence and function, to the extent that there appears to be no conception between where orthodoxy ends and orthopraxy begins, or vice-versa. At the beginning of Swanson's Móhēzhǐguān, in the introduction I believe, he talks about Ven Zhìyǐ's comparison of orthodoxy (wisdom) and orthopraxy (practice) two the two wheels of a cart or the two wings of a bird.
Nichiren Buddhism seems to take this to heart to a high degree. Hence my usage of the term "orthopraxy" rather than just any praxy. There is a "definitive" praxy in Nichiren Buddhism to match the "definitive" doxy. Like the two wings of a bird in Ven Zhìyǐ's simile. If the left wing is singular so to much the right be, perhaps, is the reasoning? Imagine a bird flying with 4 left wings and only 1 right one. One wisdom: NMRK. One practice: NMRK.
I am an outsider looking in, though, and one only slightly exposed to the tradition at that. I'm not a Nichiren Buddhist myself, so my impressions and interpretations of Nichiren Buddhism are liable to be the impressions and interpretations of someone who was not ultimately convinced that this was the "only" way, on a personal or general level. This could all well be nonsense from an inside perspective.
It's the content of that one correct practice and one correct doctrine that get debated in this sub.