  Near-infrared spectroscopic surveys have uncovered a population of
short-period, blended-light spectral binaries composed of low-mass stars and
brown dwarfs. These systems are amenable to orbit determination and individual
mass measurements via astrometric monitoring. Here we present first results of
a multi-year campaign to obtain high-precision absolute astrometry for spectral
binaries using the Gemini-South and Gemini-North GMOS imagers. We measure the
complete astrometric orbits for two systems: 2M0805$+$48 and 2M1059$-$21. Our
astrometric orbit of 2M0805$+$48 is consistent with its 2-year radial velocity
orbit determined previously and we find a mass of $66^{+5}_{-14}
M_\mathrm{Jup}$ for its T5.5 companion. For 2M1059$-$21 we find a 1.9 year
orbital period and a mass of $67^{+4}_{-5} M_\mathrm{Jup}$ for its T3.5
companion. We demonstrate that sub-milliarcsecond absolute astrometry can be
obtained with both GMOS imagers and that this is an efficient avenue for
confirming and characterising ultracool binary systems.
