  We investigate the motional resonances of dual-species Coulomb crystals
comprised of $^9$Be$^+$ and $^{24}$Mg$^+$ ions held in a 4-rod linear Paul
trap. Our experimental data and simulations show that the secular motion of
such mixed crystals has rich dynamics. Their secular spectra can differ
significantly from those of pure ion crystals. We propose a simple model based
on mechanical coupling with Coulomb interactions between the two different ion
species that explains many features of the secular spectrum. Our findings
contribute to a more reliable identification of the ion species in mixed
crystals.
