The need exists for biosensing technologies capable of sensitively and accurately detecting various biomarkers. In response, the development of nanozymes is actively underway; they have advantages in stability, cost, performance, and functionalization over natural enzymes commonly used for signal amplification in sensing technologies. However, the performance of nanozymes is interdependent with factors such as shape, size, and surface functional moiety, making it challenging to perform quantitative performance comparisons based on the nanozyme material. In this study, we propose a physical synthetic approach to fabricate double-layered bimetallic nanozymes with identical shapes, sizes, and surfaces but different material compositions. These Janus nanozymes consist of a nanozymatic layer responsible for catalytic activity and a gold layer responsible for quantification and efficient surface modification. Based on their identical physicochemical properties, the synthesized double-layered bimetallic nanozymes allow, for the first time, a quantitative comparison of nanozymatic activities in terms of various kinetic parameters. We compared several candidates and found that the Ir-Au nanozyme exhibited the best performance. Subsequently, we applied this nanozyme to detect neutralizing antibodies against SARS-CoV-2 based on a surrogate virus neutralization test. The results demonstrated a limit of detection as low as 2 pg/mL and selectivity specifically toward MERS-CoV. The performance of this assay was further validated using vaccinated samples, demonstrating the potential of our approach as a cost-effective, rapid, and sensitive diagnostic tool for neutralizing antibody detection against viruses such as SARS-CoV-2.