Check out this link:
http://www.intuscany.net/guides/shopping_spacci_2.htm
It looks like you can do a combination of train + taxi to get there from Florence. In my opinion, you could do both in one day, but I don't think there's public transportation between the two; you might have to hire a taxi.
When I was there last fall, we only did "the mall", not Prada. FWIW, my boyfriend and I could have done the whole mall in about an hour. The prices were ridiculous due to the exchange rate and I didn't see much that interested me. Bargains were few and far between, as far as I was concerned...although my BF did score a Loro Piana dress shirt for $50.