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Chimeric antigen receptor (CAR)-engineered immune effector cells constitute a promising approach for adoptive cancer immunotherapy. Nevertheless, on-target/off-tumor toxicity and immune escape due to antigen loss represent considerable challenges. These may be overcome by adaptor CARs that are selectively triggered by bispecific molecules that crosslink the CAR with a tumor-associated surface antigen. Here, we generated NK cells carrying a first- or second-generation universal CAR (UniCAR) and redirected them to tumor cells with so-called target modules (TMs) which harbor an ErbB2 (HER2)-specific antibody domain for target cell binding and the E5B9 peptide recognized by the UniCAR. To investigate differential effects of the protein design on activity, we developed homodimeric TMs with one, two or three E5B9 peptides per monomer, and binding domains either directly linked or separated by an IgG4 Fc domain. The adaptor molecules were expressed as secreted proteins in Expi293F cells, purified from culture supernatants and their bispecific binding to UniCAR and ErbB2 was confirmed by flow cytometry. In cell killing experiments, all tested TMs redirected NK cell cytotoxicity selectively to ErbB2-positive tumor cells. Nevertheless, we found considerable differences in the extent of specific cell killing depending on TM design and CAR composition, with adaptor proteins carrying two or three E5B9 epitopes being more effective when combined with NK cells expressing the first-generation UniCAR, while the second-generation UniCAR was more active in the presence of TMs with one E5B9 sequence. These results may have important implications for the further development of optimized UniCAR and target module combinations for cancer immunotherapy.