Ambjörn Naeve discovered that if you put two parabolic trough concentrators in series at the right distance apart, you can get a burning point of light. Professor Naeve, of the Royal Technical Institute, in Stockholm, Sweden, has given permission to use his images here to show his device. He showed that by bending two pieces of reflective sheet material you can mimic a paraboloidal reflector. It is much easier to bend two pieces than to bend and cut and bend and cut many times to make a parabolic dish.
Here is the proof
Here is Naeve using the device to melt copper!
Here is Naeve's diagram of a solar wheelbarrow, which holds the troughs in proper alignment.
Ambjörn Naeve made a great breakthrough back in the 1970s when he discovered and worked on these concepts.
http://kmr.nada.kth.se/wiki/Main/PointFocus is his story of the discovery and how he tried to spread the knowelege.
Unfortunately, the Naeve Cross does not share one of the main advantages of simple trough cookers, their tolerance of movements of the sun parallel to the length of the trough. This can allow a trough cooker to work for hours every day without any need for tracking the sun. But movement of the sun parallel to the length of one trough of a Naeve Cross is perpendicular to the length of the other, so the Naeve Cross has to be moved continuously or frequently to track the sun, like a paraboloid.