  We study the collective two-channel dissociation dynamics of a molecular
Bose-Einstein condensate into bosonic fragments under tight harmonic
confinement. Bose-stimulated dissociation in either channel can only take place
provided that the respective trap size $l_i$ for the fragments is large with
respect to the healing length $\zeta_i$ of the atom-molecule resonance. Thus,
even when both channels are equally coupled, differences in mass or in
dynamical polarizability enable the control of the reaction outcome by
variation of the trap frequency. In particular, if $l_1>\zeta_1$ and
$l_2<\zeta_2$, only the first channel will be amplified. This behavior
demonstrate a unique feature of 'superchemistry' wherein a chemical reaction
may be controlled by the manipulation of the reaction vessel.
