  The rare multi-body decays B->K pi ll and B_s->K K ll are both important as
backgrounds to precision analyses in the benchmark modes B->K* ll and B_s->phi
ll as well as sensitive probes of flavor physics in and beyond the standard
model. We work out non-resonant contributions to B->K pi ll and B_s->K K ll
amplitudes, where l=e,mu, at low hadronic recoil in a model-independent way.
Using the operator product expansion in 1/m_b, we present expressions for the
full angular distribution. The latter allows to probe new combinations of
|Delta B|=|Delta S|=1 couplings and gives access to strong phases between
non-resonant and resonant contributions. Exact endpoint relations between
transversity amplitudes based on Lorentz invariance are obtained. Several
phenomenological distributions including those from the angular projections to
the S-, P-, D-waves are given. Standard model branching ratios for non-resonant
B->K pi ll and B_s->K K ll decays are found to be in the few 10^{-8} region,
but drop significantly if cuts around the K* or phi mass are employed.
Nevertheless, the non-resonant contributions to B->K pi ll provide the dominant
background in the B->K* ll signal region with respect to the low mass scalars.
In B_s->K K ll, the narrowness of the phi allows for more efficient background
control. We briefly discuss lepton-flavor non-universal effects, also in view
of the recent data on R_K.
