  The DES-CMASS sample (DMASS) is designed to optimally combine the weak
lensing measurements from the Dark Energy Survey (DES) and redshift-space
distortions (RSD) probed by the CMASS galaxy sample from the Baryonic
Oscillation Spectroscopic Survey (BOSS). In this paper, we demonstrate the
feasibility of adopting DMASS as the equivalent of BOSS CMASS for a joint
analysis of DES and BOSS in the framework of modified gravity. We utilize the
angular clustering of the DMASS galaxies, cosmic shear of the DES
METACALIBRATION sources, and cross-correlation of the two as data vectors. By
jointly fitting the combination of the data with the RSD measurements from the
BOSS CMASS sample and Planck data, we obtain the constraints on modified
gravity parameters $\mu_0 = -0.37^{+0.47}_{-0.45}$ and $\Sigma_0 =
0.078^{+0.078}_{-0.082}$. We do not detect any significant deviation from
General Relativity. Our constraints of modified gravity measured with DMASS are
tighter than those with the DES Year 1 redMaGiC galaxy sample with the same
external data sets by $29\%$ for $\mu_0$ and $21\%$ for $\Sigma_0$, and
comparable to the published results of the DES Year 1 modified gravity analysis
despite this work using fewer external data sets. This improvement is mainly
because the galaxy bias parameter is shared and more tightly constrained by
both CMASS and DMASS, effectively breaking the degeneracy between the galaxy
bias and other cosmological parameters. Such an approach to optimally combine
photometric and spectroscopic surveys using a photometric sample equivalent to
a spectroscopic sample can be applied to combining future surveys having a
limited overlap such as DESI and LSST.
