f = x**2 + y**2
plot3d((f, (x, -3, 3), (y, -3, 3)),
    (-f, (x, -5, 5), (y, -5, 5)))
# Expected:
## Plot object containing:
## [0]: cartesian surface: x**2 + y**2 for x over (-3, 3) and y over (-3, 3)
## [1]: cartesian surface: -x**2 - y**2 for x over (-5, 5) and y over (-5, 5)