There are two plasma-structuring mechanisms that are known to operate at night in the midlatitude ionosphere, the Perkins instability (Perkins, 1973) in the F region, and the more recently discovered sporadic-E layer (EsL) instability (Cosgrove and Tsunoda, 2002) in the E region. Both are layer processes, whose coupled response has been shown to be more unstable than either by itself (Cosgrove and Tsunoda, 2004). Derived growth rates, however, were given only in terms of the most unstable mode; exactly how the individual growth rates in the Es and F layers are affected by coupling remains to be shown. To do so, simplified descriptions of the instabilities are derived in this paper to show that the growth rates of both layer instabilities are enhanced by additional vertical drifts produced by polarization electric fields that map between regions along geomagnetic field lines. Key findings are (1) largest enhancement is in the growth rate of F region structure (γPc), (2) γPc is controlled by the EsL instability, and (3) additional positive feedback to the EsL instability occurs because of its dependence on the field-line-integrated Pedersen conductivity of the F layer. An important byproduct of this study is the finding that a more consistent interpretation of existing data is possible, if we assume that the coupled system involves a large-scale, but finite-sized, Es patch. These new findings seem to explain several puzzling observations that are associated with midlatitude spread F.