  The mixing of $S-D$ partial waves for the heavy meson pairs in the decays
$\Upsilon(5S) \to [B^* \bar B+ {\rm c.c.}] \pi$ and $\Upsilon(5S) \to B^* \bar
B^* \pi$ is considered. It is argued that this mixing, enhanced by the heavy
meson mass, is calculable as dominated by a rescattering through pion exchange
if the production of the heavy mesons is dominated by $S$-wave `molecular'
resonances $Z_b(10610)$ and $Z_b(10650)$. The effect of the mixing grows with
the c.m. energy $E$ in each channel over the threshold, and may reach $10 \div
20$% in the rate at the upper end of applicability of the discussed approach,
$E \sim 15 $MeV. It is also argued that the mixing is likely to reach maximum
at energy approximately equal to the mass gap between the thresholds.
