Sensitivity and selectivity are critical parameters to evaluate the performance of sensors. For trace detection, it remains a challenge to design a new sensor that achieves high sensitivity and selectivity simultaneously. Here, we present a target-activated dual Mg²⁺-dependent DNAzyme (MNAzyme) that served as a simple sensing model to explore the multivalency in improving the analytical sensitivity and selectivity for target detection. Mercury ion (Hg²⁺), a notorious toxic metal ion, was selected as a model target. In the presence of Hg²⁺, the thymine-rich regions of the hairpin probe and primer could hybridize to form a stable duplex via the thymine-Hg²⁺-thymine structure. Then, an intact enzyme sequence was exposed and two separate enzyme fragments were close to each other, generating a dual MNAzyme. Benefiting from the localized high-concentration of the enzyme strand, the dual MNAzyme showed a remarkable improvement in binding stability. The catalytic rate constant of the dual MNAzyme was theoretically 1.60 times higher than that of the monomeric counterpart, and the sensitivity and selectivity had 4.50 and 1.44-fold enhancement, respectively. When the dual MNAzyme was used for sensor applications, the limit of detection was determined to be 0.04 and 0.2 nM via UV–vis spectrophotometer and naked eye, respectively. Meanwhile, the method offered desirable selectivity toward Hg²⁺ against other metal ions. With the advantages of simple operation, high sensitivity, and desirable selectivity, the developed multivalent sensing platform could be easily expanded in the future for the on-site detection of other low-abundance analytes.