I always wondered what this really meant; what words were being translated. Evidently, the phrase was used by Nichiren Shonin to describe the kind of unity he wanted among his followers. I do not know of any usage of this exact term in other forms of Buddhism. As far as I know, there is no exact equivalent concept in Pali or Sanskrit.

異體 {yiti/itai} is used as a translation of anya or anyatva; meaning different or other.  The word 異 does not mean many; it means different in the sense of odd, not uniform, irregular, or diverse. 體 {tai} means form, body, or style; but is not used to translate either rupa {form} or kaya {body}. It is the same kanji used in 如是體 {nishiti / nyozetai}; meaning the “the thusness substance of phenomena.”

同心 {tongxin/doshin} is a common compound word meaning common wishes, or a cooperative spirit. There is a literal back translation to Sanskrit; Samana-citta. Samana means something like common, sameness, or equality. It is probably Sam = same + a = toward + na = ing. Citta refers to the mind, thought, and spirituality.

For now I like; ‘diversity of styles with a common purpose.’ Of course, that only has value if we have the right spiritual purpose, that of waking up, of attaining enlightenment. I suspect the main point is to overcome our attachments to familiar modes or styles; and instead, focus on common spiritual values. The Lotus Sutra describes many different styles of Bodhisattvas emerging from Underground. This is from the Watson translation:

Each one of these bodhisattvas was the leader of his own great assembly, and each brought with him a retinue equal in number to the sands of sixty thousand Ganges. To say nothing of those who brought retinues equal to the sands of fifty thousand, forty thousand, thirty thousand, twenty thousand, or ten thousand Ganges. Or a retinue equal to as little as the sands of one Ganges, half a Ganges, one fourth of a Ganges, or as little as one part in a thousand, ten thousand, a million nayutas of Ganges. Or those whose retinue was only one thousand ten thousand million nayutas. Or only a million ten thousand. Or only a thousand ten thousand, a hundred ten thousand, or just ten thousand. Or only one thousand, one hundred, or ten. Or who brought with them only five, four, three, two or one disciple. Or those who came alone, preferring to carry out solitary practices. Such were they, then, immeasurable, boundless, beyond anything that can be known through calculation, simile or parable.