Ooh. Thought you were talking about the installs for both previews. I installed developer preview 2 over the first, so I didn't realize. It still doesn't sound very accurate to me.
Forgive me -- this is what I'm understanding from you:
DP1 can install onto a blank disk/partition.
DP2 can't install on a blank disk/partition. Needs to install as an update on top of DP 1 or Snow Leopard. (?)
That's right (at least from my personal experience). Lion currently uses system files and services from Snow Leopard to reduce its size.