  We present MMT spectroscopic observations of two massive galaxy cluster
candidates at redshift $z\sim0.07$ that show extended and diffuse X-ray
emission in the ROSAT All Sky Survey (RASS) images. The targets were selected
from a previous catalog of 303 newly-identified cluster candidates with the
similar properties using the intra-cluster medium emission. Using the new MMT
Hectospec data and SDSS archival spectra, we identify a number of member
galaxies for the two targets and confirm that they are galaxy clusters at
$z=0.079$ and 0.067, respectively. The size of the two clusters, calculated
from the distribution of the member galaxies, is roughly 2 Mpc in radius. We
estimate cluster masses using three methods based on their galaxy number
overdensities, galaxy velocity dispersions, and X-ray emission. The
overdensity-based masses are $(6\sim8) \rm \times10^{14}\ M_\odot$, comparable
to the masses of large clusters at low redshift. The masses derived from
velocity dispersions are significantly lower, likely due to their diffuse and
low concentration features. Our result suggests the existence of a population
of large clusters with very diffuse X-ray emission that have been missed by
most previous searches using the RASS images. If most of the 303 candidates in
the previous catalog are confirmed to be real clusters, this may help to reduce
the discrepancy of cosmological results between the CMB and galaxy cluster
measurements.
