  Starting with the spherical Jeans equation, we show that there exists a
radius where the mass enclosed depends only on the projected tangential
velocity dispersion, assuming that the anisotropy profile slowly varies. This
is well-approximated at the radius where the log-slope of the stellar tracer
profile is $-2$: $r_{-2}$. The associated mass is $M(r_{-2}) = 2 G^{-1} \langle
\sigma_{\mathcal{T}}^{2}\rangle^{*} r_{-2}$ and the circular velocity is
$V^{2}({r_{-2}}) = 2\langle \sigma_{\mathcal{T}}^{2}\rangle^{*}$. For a Plummer
profile $r_{-2} \simeq 4R_e/5$. Importantly, $r_{-2}$ is smaller than the
characteristic radius for line-of-sight velocities derived by Wolf et al. 2010.
Together, the two estimators can constrain the mass profiles of
dispersion-supported galaxies. We illustrate its applicability using published
proper motion measurements of dwarf galaxies Draco and Sculptor, and find that
they are consistent with inhabiting cuspy NFW subhalos of the kind predicted in
CDM but we cannot rule out a core. We test our combined mass estimators against
previously-published, non-spherical cosmological dwarf galaxy simulations done
in both CDM and SIDM. For CDM, the estimates for the dynamic rotation curves
are found to be accurate to $10\%$ while SIDM are accurate to $15\%$.
Unfortunately, this level of accuracy is not good enough to measure slopes at
the level required to distinguish between cusps and cores of the type predicted
in viable SIDM models without stronger priors. However, we find that this
provides good enough accuracy to distinguish between the normalization
differences predicted at small radii ($r \simeq r_{-2} < r_{\rm core}$) for
interesting SIDM models. As the number of galaxies with internal proper motions
increases, mass estimators of this kind will enable valuable constraints on
SIDM and CDM models.
