We demonstrate an optomechanical magnetometer where a high-Q membrane mechanical resonator responds to the magnetic-field-induced deformations of two magnetostrictive materials and is monitored by a two-beam Fabry-Perot cavity. The magnetometer enables simultaneous measurements of DC and AC magnetic fields and can recognize the direction of a two-dimensional DC magnetic field by demodulating the frequency shifts of two nondegenerate modes of the membrane. Benefiting from the two-beam interference and high-Q mechanical resonance, this device does not suffer from the linewidth and polarization fluctuation of the probe light as the reported counterparts, overcoming the dependence of optomechanical magnetometers on ultranarrow-linewidth laser and high-Q optical cavity with frequency locking equipment. This work opens a new avenue for measurements of weak magnetic fields.