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As Phil says, yes pretty much (genuinely intrigued to know what you thought they were for).
I've never bothered with a second cable though.
When I use one I usually manage to get a D-lock through the frame, wheel and round whatever I'm locking it do in one go, then loop the cable through a wheel and thread one end of it through the other and link the end up to the D-lock.This must be the most commonly derailed thread on the forum.
The way that is locked seems like it would take one chunk from the D-lock to nick the whole thing. Or is that how those extension cables are supposed to be used?