  Co-magnetometers have been validated as valuable components of the atomic
physics toolbox in fundamental and applied physics. So far, the explorations
have been focused on systems involving nuclear spins. Presented here is a
demonstration of an active alkali-metal (electronic) system, i.e. a dual
frequency spin maser operating with the collective caesium F=3 and F=4 spins.
The experiments have been conducted in both magnetically shielded and
unshielded environments. In addition to the discussion of the system's positive
feedback mechanism, the implementation of the dual frequency spin maser for
industrial non-destructive testing is shown. The stability of the F=3 and F=4
spin precession frequency ratio measurement is limited at the $3 \times
10^{-8}$ level, by the laser frequency drift, corresponding to a frequency
stability of 1.2 mHz for $10^4$ sec integration time. We discuss measurement
strategies that could improve this stability to nHz, enabling measurements with
sensitivities to the axion-nucleon and axion-electron interactions at the
levels of $f_a/C_N \sim 10^9$ GeV and $f_a/C_e \sim 10^8$ GeV, respectively.
