The reaction between tetrakis(ethylamino)silane, Si(NHEt)₄ (1) and trialkylaluminium compounds AlR₃ [R = Me (2), R = Et (3)] results in partial deprotonation of 1 and subsequent formation of a new dimeric alumosilazane in high yield. Single-crystal X-ray analyses reveal the structure of two new four-membered ring compounds [RAl(μ-NHEt)(μ-NHEt)(μ-NEt)₂Si-(NHEt)]₂ [R = Me (4), R = Et (5)]. They are isostructural and crystallize in the triclinic space group P1̄ (No. 2) with a = 8.3052(8) Å, b = 9.8323(9) Å, c = 10.324(1) Å, α = 115.498(4)°, β = 97.684(4)°, γ = 94.382(4)° (4) and a = 8.2319(3) Å, b = 9.3748(5) Å, c = 10.8221(6) Å, α = 107.035(2)°, β = 104.435(2)°, γ = 94.375(2)°, (5). The alternating edge- and corner-sharing SiN₄- and AlCN₃-tetrahedra represent a new structural motif with μ-NHEt and μ-NEt groups in one double bridge. The compounds were further characterized by elemental analysis, mass spectrometry, and NMR and IR spectroscopy.