Site-specific chemical modification of proteins can assist in the study of their function. Furthermore, these methods are essential to develop biologicals for diagnostic and therapeutic use. Standard protein engineering protocols and recombinant expression enable the production of proteins with short peptide tags recognized by enzymes capable of site-specific modification. We report here the application of two enzymes of orthogonal specificity, sortase A and butelase 1, to prepare non-natural C-to-C fusion proteins. Using these enzymes, we further demonstrate site-selective installation of different chemical moieties at two sites in a full-size antibody molecule.