Two novel isomorphic borates, Q(18)Mg(6)(B5O10)(3)(B7O14)(2)F (Q = Rb and Cs), were synthesized from high-temperature solutions. These borates are created from two kinds of isolated polyborate anionic groups, [B5O10] and [B7O14], and are further examples of two kinds of isolated polyborate anions coexisting in one borate structure. Although the title compounds are new structures, they are similar to the structures of the KBe2BO3F2 (KBBF) series of compounds, for example, Na2Be4B4O11 and gamma-KBe2B3O7, which can be regarded as the substitution of [BO3] and [B2O5]/[B3O6] groups with [B7O14] and [B5O10] groups, respectively. It is found that the coexisting B-O groups tend to have similar structures. Also, the isolation of [B5O10] and [B7O14] in the title compounds mainly derives from the special role of magnesium tetrahedra, which has a similar role to that of the BeO4 tetrahedra in the KBBF series compounds. Electron-density maps and bond-overlap population (BOP) were calculated to illustrate this feature.