Two first cousins of my mother had autosomal DNA tests done. One was the child of a full sibling of her mother. The other was the child of a full sibling of her father. While I can use them to help sort out my maternal matches, I cannot assume that all of my maternal DNA matches will match one or the other of them. It’s very possible that I have DNA from one of my maternal ancestors that the two first cousins of my mother do not.
And someone else could match me on that DNA.
The two first cousins of my mother should help me filter out a significant number of my maternal matches, but they will not help me sort out all of them.