The present invention relates to heterodimerically-tethered bispecific protein complexes (according to the general formula of (A-X)2:(Y-B)2) and libraries/multiplexes thereof for use in research and therapy and in particular an in vitro/ex vivo method of detecting synergistic biological function of otherwise unknown pairs of targets. Such complexes may be used to capture soluble molecules secreted from a particular cell, in therapy, in research and for experimental purposes such as in assays to characterise patient populations by identifying cell populations relevant to a pathology or prognosis.