Described are methods for producing libraries of cells expressing at least two separate single polypeptide chain binding proteins, in which the binding proteins have different target epitopes. Such libraries are made by integration of the nucleic acid sequences encoding the polypeptide chains into the genome of the host cell, and selecting for cells that have successfully integrated these nucleic acids. The selected cells are preferably subjected to a cloning step. Mixtures of binding proteins are produced without having to individually produce each of the components of the mixture. A library of cells wherein essentially each cell encodes at least two single polypeptide chain binding proteins having different target epitopes is also herewith provided, as well as methods for producing a composition comprising at least two separate single polypeptide chain binding proteins having different target epitopes.