CAR NK cells are a promising avenue for immunotherapy that intrinsically resolve many of the major setbacks to CAR T cell therapy, but have nonetheless failed to reach the level of consistent clinical efficacy seen with CAR T cells. CAR NK cells traditionally express a single engineered construct that utilizes one or more intracellular signaling domains to drive antigen-specific functionality, similar to the standard model of T cell activation. The activation of NK cells in situ, however, is not usually driven by the triggering of a single or pair of receptors; rather, it is governed by a suite of activating receptors working in tandem. We have therefore developed a series of novel CAR NK cells that utilize two distinct CAR constructs co-expressed by the same cell in order to better mimic natural NK cell activation. These Co-Activating Tandem (CAT)-CAR NKs contain a primary CAR optimized for cytotoxic function alongside a secondary CAR that uses a separate promoter and a different signaling domain. The intracellular domains of these secondary CARs were selected for their ability to induce NK cell proliferation and cytokine production–two features that are correlated with clinical efficacy. We tested the functionality of all possible combinations of two primary CARs and six secondary CARs against a plethora of tumor cell lines in vitro and identified several CAT-CAR NKs with significantly increased antitumor functionality compared to both the primary CARs alone and to previously-published single CAR constructs. We also compared the functionality of CAT-CAR NKs in which both the primary and secondary CARs engaged the same tumor antigen to those in which the two CARs targeted separate antigens. Our work demonstrates the success of combining multiple CAR constructs within the same cell to optimize NK cell functionality, providing a powerful new tool for NK cell-based immunotherapies.