The heterogeneity of solid tumours is a significant obstacle to the response and long-term remission of patient malignancies following Chimeric Antigen Receptor (CAR) T-cell immunotherapy in the clinic. While long-term remission has been achieved against haematological cancers, relapses have frequently occurred several months post-treatment due to outgrowing target antigen-negative tumour cells, and solid tumour responses have been less effective. Therefore, to improve solid tumour elimination and prevent relapse, CAR T-cell immunotherapy may be improved by the targeting of multiple tumour-associated antigens through dual-specific CAR T-cells, in which T-cells are engineered to express CARs against multiple antigens. Existing studies have observed significant improvements over single-specific CAR T-cells; however, few studies have interrogated the underlying biology. Therefore, we have characterised dual-targeting CAR T-cells against the Her2 and EGFRvIII tumour antigens in vitro and have generated an immunocompetent mouse model to test these constructs in vivo. Utilising functional in vitro assays to measure cytotoxic function, we have demonstrated that pooled single-specific and dual-specific CAR T- cells exhibit enhanced killing of heterogeneous target tumour cells compared to single-specific CAR T cells alone. Using CRISPR technology, we have also generated an immunocompetent mouse model tolerant to our antigens of interest, Her2 and EGFRvIII, and demonstrated the subcutaneous growth of these antigen-expressing tumours. This model allows us to evaluate the efficacy of dual-targeting CAR T cells against heterogeneous tumours in a mouse model with an intact immune system.