Although many models have been proposed , the physical mechanisms responsible for the formation of low-mass brown dwarfs are poorly understood . The multiplicity properties and minimum mass of the brown-dwarf mass function provide critical empirical diagnostics of these mechanisms . We present the discovery via gravitational microlensing of two very low-mass , very tight binary systems . These binaries have directly and precisely measured total system masses of 0.025 M _ { \odot } and 0.034 M _ { \odot } , and projected separations of 0.31 AU and 0.19 AU , making them the lowest-mass and tightest field brown-dwarf binaries known . The discovery of a population of such binaries indicates that brown dwarf binaries can robustly form at least down to masses of \sim 0.02 M _ { \odot } . Future microlensing surveys will measure a mass-selected sample of brown-dwarf binary systems , which can then be directly compared to similar samples of stellar binaries .