This study built symmetric, IgG-like bispecific antibodies (bsAbs) by fusing single-domain antibodies (sdAbs) onto full-length IgG1 scaffolds, and systematically compared how format geometry (N- vs C-terminal fusion on the heavy or light chain) affects expression, stability and antigen binding across a panel of symmetric αPD-L1×αHER2 antibodies. Flow Induced Dispersion Analysis (FIDA) characterized in-solution binding: by measuring the hydrodynamic radius of a fluorescently labeled antigen, it detected the size increase on complexation, confirmed the bsAbs bind each target individually and both simultaneously, and quantified binding affinity. More broadly, it demonstrates FIDA as an immobilization-free, in-solution way to confirm dual-antigen binding and measure binding affinity of bispecific and multispecific antibody formats.