I will tell you what I know (which isn't much):
As far as I know, there is only one list. I called the Dean and she explained the process to me and didn't mention anything about 2 lists. She said that the alternate list gets ranked in March and people get pulled off it periodically.
I do know for a fact that at least one student of the class of 2007 was put on the alternate list in March and was then deemed a "provisional admit". The meant that if she did not get in this august, then she was automatically in for next august. I do not know the process behind this, but it would be really interesting to find out.
YF