Hydrothermal reactions of Pb^II or Ba^II ion with N,N-dimethylaminomethane-1,1-diphosphonic acid (H₄L) afforded a double-chained lead(II) diphosphonate and a double-layered barium(II) diphosphonate, namely, [Pb(H₂L)] (1) and [Ba₂(H₂L)₂] (2). The two closely linked phosphonate moieties in the diphosphonate ligand exhibit a highly synergic coordination effect, which is mainly promoted by the large ionic radius of the Pb^II or Ba^II ion. More interestingly, the unusual double-layer structure in 2 contains cubane-like Ba₈ cages and each cage is face-shared with four adjacent ones.