There's not really any kind of special rig that needs to be created. I don't know if this was your impression, but just to clarify, the light instrument used to augment the practical is not hidden behind the practical from the camera's point of view. It will be out of frame behind the practical from the perspective of the subject, as close to the axis of the practical as possible without the practical itself blocking the light from hitting the subject. It is usually necessary to flag the light from your film light off of the shade of the practical, as the practical is meant to appear to be the source of light in the frame and to have light shining on your light source is pretty silly and gives the whole thing away. Sometimes you might want to dim your film light down a bit to warm up the color temperature to match the practical better.
With newer cameras that are getting better at shooting at high ISO levels, it is becoming more possible to actually light a scene with practicals. You may need to replace bulbs with higher wattages (making sure the sockets are rated accordingly) and treat the practical shades with ND gel so that the light that spills out the top and bottom of the shade are within your exposure range.
Edited by Toby Orzano, 13 February 2014 - 05:23 PM.