  Evolutionary models of pre-main sequence stars remain largely uncalibrated,
especially for masses below that of the Sun, making each new dynamical mass and
radius measurement a valuable test of theoretical models. Stellar mass
dependent features of star formation (such as disk evolution, planet formation,
and even the IMF) are fundamentally tied to these models, which implies a
systematic uncertainty that can only be improved with precise measurements of
calibrator stars. We present the discovery that UScoCTIO 5, a known
spectroscopic binary (P = 34 days, Mtot sin(i) = 0.64 Msun), is an eclipsing
system with both primary and secondary eclipses apparent in K2 light curves
obtained during Campaign 2. We have simultaneously fit the eclipse profiles
from the K2 light curves and the existing RV data to demonstrate that UScoCTIO
5 consists of a pair of nearly identical M4.5 stars with M_A = 0.329 +/- 0.002
Msun, R_A = 0.834 +/- 0.006 Rsun, M_B = 0.317 +/- 0.002 Msun, and R_B = 0.810
+/- 0.006 Rsun. The radii are broadly consistent with pre-main sequence ages
predicted by stellar evolutionary models, but none agree to within the
uncertainties. All models predict systematically incorrect masses at the
25--50% level for the HR diagram position of these mid-M dwarfs, suggesting
significant modifications for any trend of other properties that vary with
stellar mass. The form of the discrepancy for most model sets is not that they
predict luminosities that are too low, but rather that they predict
temperatures that are too high, suggesting that the models do not fully
encompass the physics of energy transport (via convection and/or missing
opacities) and/or a miscalibration of the SpT-Teff scale. The simplest
modification to the models (changing Teff to match observations) would yield an
older age for this system, in line with the recently proposed older age of
Upper Scorpius (~11 Myr).
