Actually theres more to it than the divided plenum, you can have split plenum single planes as well. Dual planes are what they are because one side of the plenum has a higher floor than the other side, aka different planes. Single planes all 8 runners feed off the same level within the plenum.
Single planes are also perfered for forced induction setups, as well as large nitrous shots when using a plate (you can direct inject it and it doesn't matter).
'72 Mach 1
"Thats not a leak, my car's marking its territory!"
"If you've done it, it ain't braggin'." -Roy Rogers