An rf magnetic field of sufficient amplitude gives rise to a nonlinear absorption caused by the unstable growth of certain spin waves. We have calculated the threshold for the case in which an rf field of frequency &ohgr;0is applied orthogonally to the dc field and another rf field at frequency 2&ohgr;0is simultaneously applied parallel to the dc field.We find that under these conditions the wave number of the spin waves with the lowest instability threshold varies discontinuously with the ratio of the two rf magnetic fields. If the orthogonal rf field is relatively small, the unstable spin waves have wavelength comparable with the sample dimensions. In this region the critical parallel field decreases linearly with the square of the orthogonal field.