yay - here you go.
they are the same as single straps, yet double the strappage.
The purpose of double straps is to provide better foot retention to sprint racers.
As far as street use - I like mine. They are double layered with laminate in between, making them stiffer/stretch is pretty much non-existent.
riding them on the street vs velodrome - they work the same. It was more of an availability issue that "will these work better?" - I just wanted a laminated strap and the other sets I saw were of equal price. I think they are a total ddeal as they hold up very well.
I got mine for $40 as I had overstretched a cheap set of singles and I no longer wanted to F around.
For new ones - I found mine on eBay from BensCycle, or you can go here:
http://businesscycles.com/trcomp.htm#straps