As far as I understand the documentation, the following should work:
f[x_, y_] = E^(-x^2) Cos[x^2 + y^2];
g[x_, y_] = 2 - x^2 - y^2;
pic = Plot3D[{f[x, y], g[x, y]}, {x, -1, 1}, {y, -1, 1},
AspectRatio -> 1, ViewPoint -> {4, 1, 1},
Filling -> {1 -> {2}}, FillingStyle -> Opacity[0.9]]
The fact that it doesn't looks like a bug to me. The following might be a reasonable workaround:
{{a, b}, {c, d}} = {{-1, 1}, {-1, 1}};
dx = (b - a)/80;
dy = (d - c)/80;
fillEdges = Graphics3D[{Opacity[0.9], EdgeForm[],
Polygon[Join[
Table[{
{x, c, f[x, c]}, {x + dx, c, f[x + dx, c]},
{x + dx, c, g[x + dx, c]}, {x, c, g[x, c]}},
{x, a, b - dx, dx}],
Table[Reverse@{
{x, d, f[x, d]}, {x + dx, d, f[x + dx, d]},
{x + dx, d, g[x + dx, d]}, {x, d, g[x, d]}},
{x, a, b - dx, dx}],
Table[{
{a, y, f[a, y]}, {a, y + dy, f[a, y + dy]},
{a, y + dy, g[a, y + dy]}, {a, y, g[a, y]}},
{y, c, d - dy, dy}],
Table[{
{b, y, f[b, y]}, {b, y + dy, f[b, y + dy]},
{b, y + dy, g[b, y + dy]}, {b, y, g[b, y]}},
{y, c, d - dy, dy}]
]]}];
Show[{pic, fillEdges}]
Plot3D[{1, 2}, {x, 0, 1}, {y, 0, 1}, Filling -> {1 -> {2}}]
Sadly, I don't know any workaround. $\endgroup$Plot3D[{1, 2}...
what is the{1, 2}
plotting, just 1 and 2 in three space? $\endgroup$