The isosurface approach that Tim suggested is possible. Here's an example of how you'd do that. It's a bit involved and it uses quite a bit of memory:
%%Start with 2 surfaces
nx = 25;
ny = 25;
nz = 25;
[x,y] = meshgrid(linspace(-2,2,nx),linspace(-2,2,ny));
z1 = peaks(x,y);
z2 = peaks(x,y) + 5;
surf(x,y,z1);
hold on
surf(x,y,z2)
%%Create 3D grid containing distance to closest surface
[y3,x3,z3] = ndgrid(linspace(-2,2,ny),linspace(-2,2,nx),linspace(-10,15,nz));
v = zeros(size(z3));
for r=1:ny
for c=1:nx
for s=1:nz
d1 = z3(r,c,s) - z1(r,c);
d2 = z2(r,c) - z3(r,c,s);
if d1 < 0
v3(r,c,s) = d1;
elseif d2 < 0
v3(r,c,s) = d2;
else
v3(r,c,s) = min(d1,d2);
end
end
end
end
%%Create isosurface
figure
p = [patch(isosurface(x3,y3,z3,v3,0)), ...
patch(isocaps(x3,y3,z3,v3,0))];
isonormals(x3,y3,z3,v3,p(1))
set(p,'FaceColor','yellow')
set(p,'EdgeColor','none')
set(p,'FaceLighting','gouraud')
view(3)
camlight right
You need both isosurface and isocaps there. The isosurface command creates the parts inside the volume (the same parts surf created) while isocaps creates the parts along the boundary. You can also combine isocaps from that example with your surface objects like this:
figure
surf(x,y,z1);
hold on
surf(x,y,z2)
isocaps(x3,y3,z3,v3,0);
