  We present X-ray, optical, near-infrared, and radio observations of GRBs
110709B and 111215A, as well as optical and near-IR observations of their host
galaxies. The combination of X-ray detections and deep optical/near-infrared
limits establish both bursts as "dark". Sub-arcsecond positions enabled by
radio detections lead to robust host galaxy associations, with optical
detections that indicate z < 4 (110709B) and 1.8 < z < 2.7 (111215A). Using the
radio and X-ray data for each burst we find that GRB 110709B requires A_V > 5.3
mag and GRB 111215A requires A_V > 8.5 mag (z=2), among the largest extinction
values inferred for dark bursts to-date. The two bursts also exhibit large
neutral hydrogen column densities (N_H > 10^22/cm^2; z=2) as inferred from
their X-ray spectra, in agreement with the trend for dark GRBs. Finally, we
find that for both bursts the afterglow emission is best explained by a
collimated outflow with a total beaming-corrected energy of E_gamma+E_K ~ 7-9 x
10^51 erg (z=2) expanding into a wind medium with a high density (n~100-350
cm^-3 at 10^17 cm). While the energy release is typical of long GRBs, the
inferred density may be indicative of larger mass loss rates for GRB
progenitors in dusty (and hence metal rich) environments. This study
establishes the critical role of radio observations in determining the origin
and properties of dark GRBs. Observations with the JVLA and ALMA will provide a
sample with sub-arcsecond positions and robust host associations that will help
shed light on obscured star formation and the role of metallicity in GRB
progenitors.
