Some blanks do seem to have two spines. I have noticed that one of the two is deffinetly stonger than the other.
When I tie a casting blank I use the stonger of the two and will tie my guides on the opposite side of the bend. I preffer a "faster" feelling rod for most of my fishing and my experience is that this takes full advantage of the "backbone" that a particullar blank has to offer.

just my two cents