= Solution
For a single equal-density <vortex sheet> with stream speeds $0$ and $V$, the gravitational contribution vanishes. The <Kelvin-Helmholtz dispersion relation with gravity> reduces to $c^2+(V-c)^2=0$, with roots $c=V/2\pm i|V|/2$. Its positive <temporal growth rate> is $\gamma_{\mathrm{sheet}}=k|V|/2$.
Both jet parity branches have the common <growth rate> derived above, hence
$$
\boxed{\frac{\gamma_{\mathrm{jet}}}{\gamma_{\mathrm{sheet}}}
=\sqrt{1-e^{-4kL}}<1\quad(0<kL<\infty,\ V\ne0).}
$$
Equivalently, $2\sqrt s\leq1+s$ follows from $(\sqrt s-1)^2\geq0$. Equality is approached when the two interfaces are separated by many disturbance decay lengths, $kL\to\infty$. The zero-shear case has both <growth rates> zero. For small $kL$, the <growth rate> ratio is approximately $2\sqrt{kL}$: interaction between the sheets suppresses amplification relative to an isolated sheet.
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