  We point out that the decay modes B^0 --> D_s^+ D_s^- and B_s^0 --> D^+ D^-
have no factorized contribution. At quark level these dacays can only proceed
through the annihilation mechanism, which in the factorized limit give zero
amplitude due to current conservation. In this paper, we identify the
dominating non-factorizable (colour suppressed) contributions in terms of two
chiral loop contributions and one soft gluon emission contribution. The latter
contribution can be calculated in terms of the (lowest dimension) gluon
condensate within a recently developed heavy-light chiral quark model. We find
braching ratios BR(B^0 --> D_s^+ D_s^-) = 7*10^-5 and BR(B^0_s --> D^+ D^-) =
1*10^-3.
