One problem with Purchase that you need to consider is that, even though they supposedly have this post-bac program for college grads whom you'd have to assume are working, none of the science classes are offered in the evening or on weekends. So if you work a 9 to 5 job, you're out of luck. I had enough flexibility in my work to schedule it around the classes, but it's a pain in the ass. I still don't get it.
The reason Purchase probably wasn't listed as having a post-bac premed program is that it's a very informal one. Basically, you get an advisor who may or may not have time for you (although the co-chair of the program, Dr. Tillotson, is very accessible), and you get access to their premedical avisory committee who will write the LoR for you. That is a big plus. Beyond that, you're on your own.
You could take the 2 classes at Manhattanville and then switch to Purchase. As long as you take at least half (I think) of the prereqs there they'll let you interview with the committee. I'd give Purchase a call, anyway, to see what they think of your situation. Good luck.
