Modified classical trivacant Wells-Dawson α-[P₂W₁₅O₅₆]¹²⁻ and the assembly of related sandwiched transition metal clusters are of interest, but surprisingly few reports of these materials exist because of the sensitivity of α-[P₂W₁₅O₅₆]¹²⁻ to the assembly environment. Herein, we describe the pH-controlled assembly of two novel Dawson-sandwiched clusters, (H₂bpz)₆[Co₂(P₂W₁₆O₅₇)₂]·22H₂O (1, bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole) and (H₂bpz)₆[Co₃H₂(P₂W₁₆O₅₇)(P₂W₁₅O₅₆)(H₂O)]·12H₂O (2), involving the in situ transformation of α-[P₂W₁₅O₅₆]¹²⁻. Both clusters were characterized by X-ray single-crystal diffraction, FT-IR spectroscopy, UV-Visible spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and elemental analyses. X-ray crystallography showed that both heteropolytungstates become divacant α-[P₂W₁₆O₅₇]⁸⁻ and symmetrically encapsulate two edge-shared CoO₆ octahedra in the interior in 1, while only one divacant α-[P₂W₁₆O₅₇]⁸⁻ is observed in 2, which combined with another trivacant α-[P₂W₁₅O₅₆]¹²⁻ to asymmetrically clamp three edge-shared CoO₆ octahedra. The α-[P₂W₁₆O₅₇]⁸⁻ heteropolytungstate should be generated in situ from α-[P₂W₁₅O₅₆]¹²⁻via self-decomposition equilibria in solution. Their electrochemical behaviors reveal characteristic multi-electron redox processes related to W^VI centers. The electrocatalytic reduction performances toward nitrite, hydrogen peroxide, chlorate, bromate and iodate were fully measured and discussed; among these species, both clusters exhibit the best electrocatalytic activity towards the reduction of bromate. Magnetic measurements indicate weak ferromagnetic exchange interactions between Co atoms sandwiched by vacant polyoxometalates.