Question: How can I plot a 3D and a 2D graph together?

for instance: s1:=implicitplot3d(z=x*y, x=-8..8,y=-8..8,z=-8..8,axes=normal): s2:=implicitplot(x^2+y^2=1, x=-8..8,y=-8..8,axes=normal): if I type display(s1,s2); it gives an error.
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