Two-sided materials are a bit buggy with ART, so I'm not surprised you're not seeing an outline.
There is, however, a quick workaround - I'll just go over the whole process start to finish here. Select your object; copy; paste; select one copy of the object and hide it; select all the faces of the other copy; unhide the first copy; go back to point edit mode and "extrude connected" in wirefraeme mode until you have the edge thickness you want. Now the workaround: select the outline object, cut and paste it into a new object layer, apply a two-sided material to it (transparent front, another colour - not necessarily black to the other side), cut and paste it back to your first object. I find that works, but it does tend to be a bit random; especially if you do weird things like put transparency and bump maps on your outline texture.
I don't check this forum everyday, but feel free to ask any follow-up questions and I'll try to help
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