I was wondering that as well, I'll probably go with listing another undergraduate institution and putting my second major with all the other information the same. My only concern is that this may be construed as having dual degrees instead of double majors, but they have our AMCAS and either way with how they set it up, there will be some discrepancy. I'd rather have all the info included on the secondary.
For the poster directly above, 2010-2011 will be Columbia-Bassett's first year, so there are no admission statistics. You can apply to Columbia-Bassett and if you don't get in, you'll also be considered for MD-only. I'd imagine that Columbia-Bassett will be equally if not more competitive than Columbia's MD-only. Given that it will likely be construed by many as "2 chances" at Columbia, I'm sure they are going to have many applications and pretty high screening standards for this first class.