We present a novel Bayesian methodology to jointly model photometry and deep Hubble Space Telescope 2D grism spectroscopy of high-redshift galaxies. Our requiem2d code measures both unresolved and resolved stellar populations, ages, and star formation histories (SFHs) for the ongoing REsolving QUIEscent Magnified (REQUIEM) Galaxies Survey, which targets strong gravitationally lensed quiescent galaxies a z similar to 2. We test the accuracy of requiem2d using a simulated sample of massive galaxies at z similar to 2 from the Illustris cosmological simulation and find that we recover the general trends in SFH and median stellar ages. We further present a pilot study for the REQUIEM Galaxies Survey: MRG-S0851, a quintuply imaged, massive (logM/M-circle dot = 11.02 +/- 0.04) red galaxy at z = 1.883.+/-.0.001. With an estimated gravitational magnification of mu = 5.7(-0.2)(+0.4), we sample the stellar populations on 0.6 kpc physical size bins. The global mass-weighted median age is constrained to be 1.8(-0.2)(+0.3) Gyr, and our spatially resolved analysis reveals that MRG-S0851 has a flat age gradient in the inner 3 kpc core after taking into account the subtle effects of dust and metallicity on age measurements, favoring an early formation scenario. The analysis for the full REQUIEM-2D sample will be presented in a forthcoming paper with a beta release of the requiem2d code.