Lipid nanoparticles (LNPs) are a powerful emerging tool for in vivo T cell engineering to treat diseases ranging from B cell lymphomas and other cancers to autoimmune diseases. Key challenges in designing these therapeutics include achieving both precise cell targeting and efficient mRNA translation. While single-targeted LNPs have been extensively studied, bispecific LNPs have only been briefly explored. Multiple receptor engagement offers the opportunity for enhanced mRNA delivery, expression, and T cell targeting. Here, a DNA-tethering method was developed to enable rapid modification of lipid nanoparticles with commercial antibodies. Using this strategy, we evaluated a variety of bispecific LNPs for targeted mRNA delivery to T cells both in vitro and in vivo. We identify bispecific formulations that improve targeting and subsequent transfection of T cells in vitro and in vivo relative to monotargeted LNPs. Additionally, we find that targeting molecules can alter LNP biodistribution to the spleen and liver. This fast and efficient approach to assembling antibody-targeted LNPs enables high-throughput screening of diverse antibody combinations for improved specificity and efficiency of in vivo gene delivery.