  Nitrogen-Vacancy (NV) spin in diamond is a versatile quantum sensor, being
able to measure physical quantities such as magnetic field, electric field,
temperature, and pressure. However, sensing more than a single quantity
simultaneously has not been realized previously. In the present work, we
demonstrate a multiplexed sensing of magnetic field and temperature. Unlike
time-division, the dual frequency driving technique we employ here is based on
frequency-division multiplexing, which enables sensing both measurables in real
time. The pair of NV resonance frequencies for dual frequency driving must be
selected to avoid coherent population trapping of NV spin states. With an
enhanced optical collection efficiency higher than 50 % and a type 1b diamond
crystal with natural abundance $^{13}$C spins, we achieve sensitivities of
about 70 pT/Hz$^{1/2}$ and 25 $\mu$K/Hz$^{1/2}$ simultaneously with an
isolation factor of about 34 dB. This work paves the way for extending the
application of NV quantum diamond sensors into more demanding conditions.
