  We present and validate 20 deg$^2$ of overlapping synthetic imaging surveys
representing the full depth of the Nancy Grace Roman Space Telescope
High-Latitude Imaging Survey (HLIS) and five years of observations of the Vera
C. Rubin Observatory Legacy Survey of Space and Time (LSST). The two synthetic
surveys are summarized, with reference to the existing 300 deg$^2$ of LSST
simulated imaging produced as part of Dark Energy Science Collaboration (DESC)
Data Challenge 2 (DC2). Both synthetic surveys observe the same simulated DESC
DC2 universe. For the synthetic Roman survey, we simulate for the first time
fully chromatic images along with the detailed physics of the Sensor Chip
Assemblies derived from lab measurements using the flight detectors. The
simulated imaging and resulting pixel-level measurements of photometric
properties of objects span a wavelength range of $\sim$0.3 to 2.0 $\mu$m. We
also describe updates to the Roman simulation pipeline, changes in how
astrophysical objects are simulated relative to the original DC2 simulations,
and the resulting simulated Roman data products. We use these simulations to
explore the relative fraction of unrecognized blends in LSST images, finding
that 20-30% of objects identified in LSST images with $i$-band magnitudes
brighter than 25 can be identified as multiple objects in Roman images. These
simulations provide a unique testing ground for the development and validation
of joint pixel-level analysis techniques of ground- and space-based imaging
data sets in the second half of the 2020s -- in particular the case of joint
Roman--LSST analyses.
