  We discuss a nonfactorizable (NF) contribution of a charm loop to the FCNC
$B$-decay amplitude given through the three-particle Bethe-Salpeter amplitude
(3BS) of the $B$-meson. This 3BS contains one heavy-quark field and two light
fields (a light quark and a gluon). Our discussion is aimed at clarifying
properties of the $B$-meson 3BS necessary to describe properly charm-loop
contributions to the amplitudes of FCNC $B$-decays. We demonstrate that the
dominant contribution of nonfactorizable charm to FCNC $B$-decay amplitude is
given in the heavy-quark limit by a convolution of some hard kernel and the
$B$-meson 3BS in a "double-collinear" light cone (LC) configuration: one of the
light degrees of freedom $\phi(x)$, $x^2=0$, lies on the $(+)$-direction of the
LC, whereas another light degree of freedom $\phi'(x')$, $x'^2=0$ lies on the
$(-)$-direction. We show the emergence of new constraints on the distribution
amplitudes which parametrize the 3BS in this double-collinear configuration.
