  By virtue of their sub-hour orbital periods, ultra-compact X-ray binaries are
promising sources for the space-borne gravitational-wave interferometers LISA,
Taiji, and TianQin. Some of these systems contain a neutron star primary, whose
spin period can be measured directly via pulse timing, or indirectly through
rotational modulations of burst phenomena. It is pointed out here that since
actively accreting stars, with spin frequencies in the hundreds of Hz, may
continuously emit appreciable gravitational waves due to the presence of
accretion-built mountains, toroidal magnetic fields, and/or $r$-mode
oscillations, such binaries are also candidate sources for ground-based
interferometers. Two Galactic systems (4U 1728-34 and 4U 1820-30) are
identified as being potentially detectable by both LISA and aLIGO
simultaneously: a dual-line detection of this sort could provide percent-level
constraints on the mass, radius, and internal magnetic field strength of the
neutron star. With the Einstein Telescope, we find that at least four of the
known ultra-compact binaries become dual-line visible.
