Hybrid bicyclo[1.1.0]butanes comprised of different group 14 elements, namely, 1,2,3-trisilabicyclo[1.1.0]butane and 1,3-disila-2-germabicyclo[1.1.0]butane, were readily prepared by the reductive dichlorination of 1,3-dichlorocyclobutane precursors. Both compounds feature exceptionally short bridging Si–Si bonds and practically planar geometry around the bridging Si atoms, thus representing a novel structural motif in the chemistry of the heavy analogues of bicyclo[1.1.0]butanes. These peculiar structures were explained by the steric interaction between the substituents at the bridging and bridgehead positions. Thermolysis of the title bicyclo[1.1.0]butanes in toluene at 130 °C resulted in their isomerization to the alkyl-substituted cyclopropene analogues. The mechanism of this isomerization was revealed in detail on the basis of deuterium-labeled experiments, kinetic investigations, trapping experiments, and computational studies to establish the nature of the key intermediate of the whole isomerization process.