  We present a combined tomographic weak gravitational lensing analysis of the
Kilo Degree Survey (KV450) and the Dark Energy Survey (DES-Y1). We homogenize
the analysis of these two public cosmic shear datasets by adopting consistent
priors and modeling of nonlinear scales, and determine new redshift
distributions for DES-Y1 based on deep public spectroscopic surveys. Adopting
these revised redshifts results in a $0.8\sigma$ reduction in the DES-inferred
value for $S_8$, which decreases to a $0.5\sigma$ reduction when including a
systematic redshift calibration error model from mock DES data based on the
MICE2 simulation. The combined KV450 + DES-Y1 constraint on $S_8 =
0.762^{+0.025}_{-0.024}$ is in tension with the Planck 2018 constraint from the
cosmic microwave background at the level of $2.5\sigma$. This result highlights
the importance of developing methods to provide accurate redshift calibration
for current and future weak lensing surveys.
