Let C be a code of length n over an alphabet of q letters. The descendant code desc(C 0 ) of C 0 = {c 1 , c 2 , . . . , c t } ⊆ C is defined to be the set of wordsThe study of separable codes is motivated by questions about multimedia fingerprinting for protecting copyrighted multimedia data. Let M(t, n, q) be the maximal possible size of such a separable code. In this paper, we provide an improved upper bound for M(2, 2, q) by a graph theoretical approach, and a new lower bound for M(2, 2, q) by deleting suitable points and lines from a projective plane, which coincides with the improved upper bound in some places. This corresponds to the bounds of maximum size of bipartite graphs with girth 6 and a construction of such maximal bipartite graphs.