  Based on different neutron star-white dwarf (NS-WD) population models, we
investigate the prospects of gravitational-wave (GW) detections for NS-WD
mergers, with the help of early warnings from two space-borne decihertz GW
observatories, DO-Optimal and DECIGO. We not only give quick assessments of the
GW detection rates for NS-WD mergers with the two decihertz GW detectors, but
also report systematic analyses on the characteristics of GW-detectable merger
events using the method of Fisher matrix. With a sufficient one-day
early-warning time, the yearly GW detection number for DO-Optimal is in the
range of $ (1.5$-$1.9) \times 10^{3}$, while it is $ (3.3$-$4.6) \times 10^{4}$
for DECIGO. More importantly, our results show that most NS-WD mergers can be
localized with an uncertainty of $O(10^{-2})\,\mathrm{deg}^2$. Given the NS-WD
merger as a possible origin for a peculiar long-duration gamma-ray burst, GRB
211211A, followed with kilonova-like emissions, we further suggest that the GW
early-warning detection would allow future electromagnetic telescopes to get
prepared to follow-up transients after some special NS-WD mergers. Based on our
analyses, we emphasize that such a feasible "wait-for" pattern can help to
firmly identify the origin of GRB 211211A-like events in the future and bring
excellent opportunities for the multimessenger astronomy.
