  We present the first discoveries of the double-lined double white dwarf (DBL)
survey that targets over-luminous sources with respect to the canonical white
dwarf cooling sequence according to a set of well-defined criteria. The primary
goal of the DBL survey is to identify compact double white dwarf binary star
systems from a unique spectral detection of both stars, which then enables a
precise quantification of the atmospheric parameters and radial velocity
variability of a system. Our search of 117 candidates that were randomly
selected from a magnitude limited sample of 399 yielded a 29% detection
efficiency with 34 systems exhibiting a double-lined signature. A further 38
systems show strong evidence of being single-lined or potentially-double-lined
double white dwarf binaries and 7 single-lined sources from the full observed
sample are radial velocity variable. The 45 remaining candidates appear as a
single WD with no companion or a non-DA white dwarf, bringing the efficiency of
detecting binaries to 62%. Atmospheric fitting of all double-lined systems
reveals a large fraction that have two similar mass components that combine to
a total mass of 1.0-1.3 solar masses - a class of double white dwarf binaries
that may undergo a sub-Chandrasekhar mass type Ia detonation or merge to form a
massive O/Ne WD, although orbital periods are required to infer on which
timescales. One double-lined system located 49pc away, WDJ181058.67+311940.94,
is super-Chandrasekhar mass, making it the second such double white dwarf
binary to be discovered.
