How to add multiple cut planes in the Viewer?

[qweasel]

I would like to visualize calculation results (e.g. electron density) using 2 perpendicular cut planes. When I open the viewer I'm asked either isosurface or cut cut plane. Afterwards, under "Properties"->"Cut planes" I don't see any option to add a second cut plane. How can I achieve this?

[Anders Blom]

Drop the same grid onto the open 3D window a second time.