  We demonstrate a technique for precision sensing of temperature or the
magnetic field by simultaneously driving two hyperfine transitions involving
distinct electronic states of the nitrogen-vacancy center in diamond. Frequency
modulation of both driving fields is used with either the same or opposite
phase, resulting in the immunity to fluctuations in either the magnetic field
or the temperature, respectively. In this way, a sensitivity of 1.4 nT
Hz$^{-1/2}$ or 430 $\mu$K Hz$^{-1/2}$ is demonstrated. The presented technique
only requires a single frequency demodulator and enables the use of
phase-sensitive camera imaging sensors. A simple extension of the method
utilizing two demodulators allows for simultaneous, independent, and
high-bandwidth monitoring of both the magnetic field and temperature.
