  We describe one of the largest radio-frequency RF atomic magnetometers
presently operating. A total atomic volume of 128 $\mathrm{cm^3}$, with
correspondingly large number of $^{87}$Rb atoms, can reduce atom noise. A total
of 44 passes of the probe beam reduces photon-shot noise. The atomic vapor is
divided between two chambers allowing for pumping of the cells individually;
doing so with opposite-helicity light enables use as an intrinsic gradiometer.
In this configuration, common-mode noise sources including light-shift noise
can be reduced. Magnetic tuning fields can also be applied to the chambers
individually, allowing simultaneous measurement of two frequencies. An
application of this is in the search for contraband materials using Nuclear
Quadrupole Resonance (NQR), for which simultaneous measurement can
significantly reduce search times. We demonstrate dual-frequency measurement on
an effective range of 423-531 kHz, corresponding to the NQR frequencies of
ammonium nitrate NH$_4$NO$_3$ at the lowest value and potassium chlorate
KClO$_3$ at the highest. We explore fundamental, as well as instrumental, noise
contributions to the sensitivity in this system.
