  We report on the operation of co-located ${}^{129}$Xe and ${}^{131}$Xe
nuclear spin masers with an external feedback scheme, and discuss the use of
${}^{131}$Xe as a comagnetometer in measurements of the ${}^{129}$Xe spin
precession frequency. By applying a correction based on the observed change in
the ${}^{131}$Xe frequency, the frequency instability due to magnetic field and
cell temperature drifts are eliminated by two orders of magnitude. The
frequency precision of 6.2 $\mu$Hz is obtained for a 10$^4$ s averaging time,
suggesting the possibility of future improvement to $\approx$ 1 nHz by
improving the signal-to-noise ratio of the observation.
