  The longitudinal resistivity $\rho_{xx}$ of two-dimensional electron gases
formed in wells with two subbands displays ringlike structures when plotted in
a density--magnetic-field diagram, due to the crossings of spin-split Landau
levels (LLs) from distinct subbands. Using spin density functional theory and
linear response, we investigate the shape and spin polarization of these
structures as a function of temperature and magnetic-field tilt angle. We find
that (i) some of the rings "break" at sufficiently low temperatures due to a
quantum Hall ferromagnetic phase transition, thus exhibiting a high degree of
spin polarization ($\sim 50 $%) within, consistent with the NMR data of Zhang
\textit{et al.} [Phys. Rev. Lett. {\bf 98}, 246802 (2007)], and (ii) for
increasing tilting angles the interplay between the anticrossings due to
inter-LL couplings and the exchange-correlation (XC) effects leads to a
collapse of the rings at some critical angle $\theta_c$, in agreement with the
data of Guo \textit{et al.} [Phys. Rev. B {\bf 78}, 233305 (2008)].
