Symbiosis (the association of two different organisms attached to each other or one within the other for mutual advantage) derived from the Greek,
sumbiosis = "a living together", does not appear to have a one-word (or any) Latin equivalent. The Romans do not even seem to have borrowed the Greek word (Oxford; Lewis & Short; Wiki offer nothing.)
Those who love David-Attenborough's progs. will have seen birds, perched on the back of a rhinoceros, devouring the insects who have taken up residence in the rhino's skin: the rhino gets clean; the birds get fed. There are many examples in nature; but, how to express this concept, in Latin:
inter se ipsos superant bene = "They survive well amongst themselves.".
This may be too long when brevity is required:
inter duos superant = "They survive between the two (of them).".
inter duos = "between the two".
Therefore, a symbiotic relationship would be "a-between-the-two" = "