Double truss rod neck?

So I want to build a 12 string style tele and build my own neck and I want to give it double truss rods how is a double truss rod different and could I add an extra truss rod to an old neck I have?

You might just want a single rod down the middle and two graphite rods embedded on the right and left of it.
I did this on a 7 string bass I made years ago.
http://www.stewmac.com/shop/Truss_rods/Carbon_fiber/Carbon_Fiber_Neck_Rods.html

A hotrod style rod ( double rod) sits in a flat slot and a single rod sits in a slightly curved slot with a filler strip. There is no filler strip with a double rod, it sits directly below the fretboard.

You would need the truss rods to be symmetrical, otherwise there would be a risk of twisting.
I am never sure if double truss rods are ever required, even on a twelve string instrument but, that said, I have a Rickenbacker 350V63 six string model with double truss rods. Supposedly any slight twist would be able to be corrected. I have never needed to touch the rods, even though the neck is one of the thinnest I have ever played. I think the facility to fix twist would be more useful than simply conteracting the tension of the strings.