We study two-dimensional electron systems confined in wide quantum wells whose subband separation is comparable with the Zeeman energy. Two Landau levels from different subbands and with opposite spins are pinned in energy when they cross each other and electrons can freely transfer between them. When the disorder is strong, we observe clear hysteresis in our data corresponding to the instability of the electron distribution in the two crossing levels. When the intralayer interaction dominates, multiple minima appear when a Landau level is or filled and the fractional quantum Hall effect can be stabilized.