Gracindo Pereira sent in this drawing of his foam-cutting setup. It has 2 cutting wires so that both surfaces of a wing can be cut in a single pass.

I haven't tried it but it looks like it should work from a mechanical standpoint. Please note that he does not make any specific recommendations for the power supplies to the cutting wires. Unless you have previous experience working with this kind of stuff this will not be a good first project for you to tackle. It looks easy but if you mess anything up you could easily set your house or yourself aflame.