Ah, so you don't know how it works and are making things up based on certain practices. Got it.
Yes, mixing some nitrogen fixing plants with non-fixing plants can be a good way of keeping enough nitrogen in the soil. The bacteria colonies in nitrogen fixing plants can produce more fixed nitrogen than that plant needs, and their host plant can provide a better environment for them than bare soil.
None of that suggests that "a percentage of plants get it from other nitrogen fixing plants".