  The spectra of fast radio bursts (FRBs) encode valuable information about the
source's local environment, underlying emission mechanism(s), and the
intervening media along the line of sight. We present results from a long-term
multiwavelength radio monitoring campaign of two repeating FRB sources, FRB
121102 and FRB 180916.J0158+65, with the NASA Deep Space Network (DSN) 70-m
radio telescopes (DSS-63 and DSS-14). The observations of FRB 121102 were
performed simultaneously at 2.3 and 8.4 GHz, and spanned a total of 27.3 hr
between 2019 September 19 and 2020 February 11. We detected 2 radio bursts in
the 2.3 GHz frequency band from FRB 121102, but no evidence of radio emission
was found at 8.4 GHz during any of our observations. We observed FRB
180916.J0158+65 simultaneously at 2.3 and 8.4 GHz, and also separately in the
1.5 GHz frequency band, for a total of 101.8 hr between 2019 September 19 and
2020 May 14. Our observations of FRB 180916.J0158+65 spanned multiple activity
cycles during which the source was known to be active and covered a wide range
of activity phases. Several of our observations occurred during times when
bursts were detected from the source between 400-800 MHz with the Canadian
Hydrogen Intensity Mapping Experiment (CHIME) radio telescope. However, no
radio bursts were detected from FRB 180916.J0158+65 at any of the frequencies
used during our observations with the DSN radio telescopes. We find that FRB
180916.J0158+65's apparent activity is strongly frequency-dependent due to the
narrowband nature of its radio bursts, which have less spectral occupancy at
high radio frequencies ($\gtrsim$ 2 GHz). We also find that fewer or fainter
bursts are emitted from the source at high radio frequencies. We discuss the
implications of these results on possible progenitor models of repeating FRBs.
