  We present in this paper a Herriott-cavity-assisted closed-loop Xe isotope
gas comagnetometer. In this system, $^{129}$Xe and $^{131}$Xe atoms are pumped
and probed by polarized Rb atoms, and continuously driven by oscillating
magnetic fields, whose frequencies are kept on resonance by phase-locked loops
(PLLs). Different from other schemes, we use a Herriott cavity to improve the
Rb magnetometer sensitivity instead of the parametric modulation method, and
this passive method is aimed to improve the system stability while maintaining
the sensitivity. This system has demonstrated an angle random walk (ARW) of
0.06 $^\circ$/h$^{1/2}$, and a bias instability of 0.2 $^\circ$/h (0.15
$\mu$Hz) with a bandwidth of 1.5 Hz. By adding a closed-loop Rb isotope
comagnetometer, we can extend this system to dual simultaneously working
comagnetometers sharing the same cell. This extended system has wide
applications in precision measurements, where we can simultaneously and
independently measure the coupling of anomalous fields with proton spin and
neutron spin.
