The highlighted region is under the plots of x[t] and y[t], so the plots are not covered by the plots like Epilog did.
In:
Remove[x, y, t, highlight]
x[t_] = Sin[50 t]/t + Cos[t];
y[t_] = Sin[30 t]/t^2 + Cos[t^2];
P1 = Plot[x[t], {t, 0, 2 \[Pi]}, PlotStyle -> Blue];
P2 = Plot[y[t], {t, 0, 2 \[Pi]}, PlotStyle -> Red];
HighlightRegion[f1_, f2_, a_, b_] :=
Module[{t, xyMin, xyMax, highlight, fs},
fs = {f1, f2};
xyMin =
fs // Map[NMinValue[{#[t], a <= t <= b}, t] &] // {a, Min[#]} &;
xyMax = fs //
Map[NMaxValue[{#[t], a <= t <= b}, t] &] // {b, Max[#]} &;
highlight = Graphics[{Yellow, Rectangle[xyMin, xyMax]}, Axes -> True]
]
highlight = HighlightRegion[x, y, 1, 2];
Show[highlight, P2, P1]
Out:
