When I traveled to France last year, I used the eurail pass... it's for unlimited travel on the rail networks! They're pricey, but using them is cheaper [or about the same price] as buying a one-way ticket from Florence to Paris. Since there are two of you, you can use the "saver" option, where you get a really nice discount on the ticket.
Eurail - France - Italy Pass
My aunt uses their global pass every time she goes to Europe... they travel all over the place. I'm so jealous of her life!
Unfortunately, I don't know how the sleeper cars work... I'm sure that there is information on their website that you can find.