Anti-CD19 chimeric antigen receptor T (CAR T) cells have promising therapeutic potential for diffuse large B-cell lymphoma (DLBCL), yet many treated patients relapse due to progressive disease driven by CD19-negative clones. Tandem CAR (TCAR) designs co-targeting CD19 and CD20 may overcome this problem. However, murine and primate models do not fully recapitulate human disease and clinical responses to CAR T therapy, limiting preclinical optimization. Moreover, clinical trials are lengthy and costly, further delaying the evaluation of new strategies. By contrast, canine DLBCL closely resembles the human disease, and CAR T trials in canines are feasible, with clinical sequelae that mirror those observed in human CAR T therapy. We previously reported CD20 loss in canine DLBCL patients treated with CD20-specific CAR T cells, consistent with mechanisms of antigen escape in humans. We therefore hypothesized that canine DLBCL could be leveraged to accelerate the translation of more effective TCAR-based strategies, benefiting both canine and human patients. To test this, we first assessed the expression of CD19 and CD20 in canine lymphoma patient samples. We then developed canine TCARs directed against CD19 and CD20 and evaluated their efficacy and specificity against canine DLBCL cells. We show that canine B-cell lymphoma co-expresses CD19 and CD20 with heterogeneous expression patterns similar to those observed in humans, and that TCAR-engineered canine T cells effectively and specifically eliminate cells expressing CD19 and/or CD20. Our TCAR platform holds promise to improve outcomes in canine DLBCL and to further optimize next-generation CAR-based strategies before entering human trials.