  Recently, Tamanini & Danielski (2019) discussed the possibility to detect
circumbinary exoplanets (CBPs) orbiting double white dwarfs (DWDs) with the
Laser Interferometer Space Antenna (LISA). Extending their methods and
criteria, we discuss the prospects for detecting exoplanets around DWDs not
only by LISA, but also by Taiji, a Chinese space-borne gravitational-wave (GW)
mission which has a slightly better sensitivity at low frequencies. We first
explore how different binary masses and mass ratios affect the abilities of
LISA and Taiji to detect CBPs. Second, for certain known detached DWDs with
high signal-to-noise ratios, we quantify the possibility of CBP detections
around them. Third, based on the DWD population obtained from the Mock LISA
Data Challenge, we present basic assessments of the CBP detections in our
Galaxy during a 4-year mission time for LISA and Taiji. We discuss the
constraints on the detectable zone of each system, as well as the distributions
of the inner/outer edge of the detectable zone. Based on the DWD population, we
further inject two different planet distributions with an occurrence rate of
$50\%$ and constrain the total detection rates. We finally briefly discuss the
prospects for detecting habitable CBPs around DWDs with a simplified model.
These results can provide helpful inputs for upcoming exoplanetary projects and
help analyze planetary systems after the common envelope phase.
