  Accurate measurements of the cross-plane thermal conductivity {\Lambda}_cross
of a high-thermal-conductivity thin film on a low-thermal-conductivity
({\Lambda}_s) substrate (e.g., {\Lambda}_cross/{\Lambda}_s>20) are challenging,
due to the low thermal resistance of the thin film compared to that of the
substrate. In principle, {\Lambda}_cross could be measured by time-domain
thermoreflectance (TDTR), using a high modulation frequency f_h and a large
laser spot size. However, with one TDTR measurement at f_h, the uncertainty of
the TDTR measurement is usually high due to low sensitivity of TDTR signals to
{\Lambda}_cross and high sensitivity to the thickness h_Al of Al transducer
deposited on the sample for TDTR measurements. We observe that in most TDTR
measurements, the sensitivity to h_Al only depends weakly on the modulation
frequency f. Thus, we performed an additional TDTR measurement at a low
modulation frequency f_0, such that the sensitivity to h_Al is comparable but
the sensitivity to {\Lambda}cross is near zero. We then analyze the ratio of
the TDTR signals at f_h to that at f_0, and thus significantly improve the
accuracy of our {\Lambda}cross measurements. As a demonstration of the
dual-frequency approach, we measured the cross-plane thermal conductivity of a
400-nm-thick nickel-iron alloy film and a 3-{\mu}m-thick Cu film, both with an
accuracy of ~10%. The dual-frequency TDTR approach is useful for future studies
of thin films.
