Computations of goup velocity of whistlers in a two-ion plasma show that a quasi-longitudinal (QL) right-handed polarized wave (electron wave) has a minimum group velocity at the so-called crossover frequency; a QL left-handed polarized wave has a maximum group velocity at the ion hybrid frequency. In an oxygen/hydrogen plasma this minimum occurs only for almost perpendicular waves, whereas in a helium/hydrogen plasma it occurs also for waves propagating off perpendicular to the magnetic field. We also find that a small concentration of helium in a hydrogen plasma introduces a 'stop band' for quasi-transverse waves with frequencies just above the helium gyrofrequency. The theoretical results are used to interpret satellite observations of quasi-tranverse low-frequency whistlers.