  We study effects of tilted magnetic fields on energy levels in a
double-quantum-well (DQW) system, focusing on the coupling of subbands and
Landau levels (LLs). The subband-LL coupling induces anticrossings between LLs,
manifested directly in the magnetoresistance. The anticrossing gap becomes
larger than the spin splitting at the tilting angle $\theta \sim 20^\circ $ and
larger than the cyclotron energy at $\theta \sim 50^\circ $, demonstrating that
the subband-LL coupling exerts a strong influence on quantum Hall states even
in at a relatively small $\theta $ and plays a dominant role for larger $\theta
$. We also find that when the DQW potential is asymmetric, LL coupling occurs
even within a subband. Calculations including higher-order coupling reproduce
the experimental results quantitatively well.
