  A study of hadron pair production mechanism is motivated by the recent
observed decays $\bar B^0\to D^{(*)+}K^-K^0$. One novel phenomenon is threshold
enhancement of the kaon pair production. We show that these decays in the heavy
quark mass limit can be factorized into a generalized form. The new
non-perturbative quantity is the generalized distribution amplitude which
describes how a quark-antiquark pair transmits into the hadron pair. A proof of
factorization of $\bar B^0\to D^{(*)+}K^-K^0$ decays to all-orders is performed
by using the soft-collinear effective theory. The phenomenological application
is discussed in brief.
