  We present a study of the constraining power of low-threshold,
high-resolution direct-detection experiments for a biportal simplified dark
matter model. In the scenario we consider here, dark matter and Standard Model
particles interact through two dark vector mediators $-$ one light and one
heavy with respect to the momentum transfer in the experiment. Interference
effects at the level of scattering amplitudes can lead to novel marked shape
features in the differential recoil spectra, which are best exploited by
high-resolution, low-threshold experiments. We identify the region in
parameters space for our model where such effects are dominant and show that
composite-target experiments with large atomic mass differences are ideal to
explore these scenarios. We develop a profile likelihood approach to analyze
presently available and future data. Using published results by the CRESST-III
experiment and projections of future sensitivities for the COSINUS experiment,
we constrain the parameter space in our model, thereby showing the potential of
such an analysis on a class of dark matter models which exhibit non-standard
features in the recoil spectra.
