Others Ziners pointed out the problem about the seat reservation that is definitely a issue and you have to be aware, anyway about get on and off from trains, the rules depends on how long is your journey. If it is less than some 100's km (I don't remember how much exactely), your journey have to finish by 6 hours from the validation. If your ticket is for a longer journey, it must finish by 24 hours (or cross the border by 24 h). Only for the long journey tickets, you can get off the train for 1 day and catch again the train and arriving at your destination by 48 hours from the validation at the condition to make a second validation when you get on the train again. The validity of your ticket (6 or 24 hours) is written on the ticket itself.

I'm afraid I'm not very clear, I'll try to summarize. You can get on and off as many time as you like by the validity of your ticket (6 or 24 hours, depending on how distant is your final destination) during your journey. With a 24 hours ticket, you have the chance to validate a second time your ticket if your journey takes more than 24h (over stop included), but it must finish by 48h from the first validation. Anyway the trains rules change by month to month.

That is for clarification to all the Ziners interested in the subject. Now, about you, if you plan to break your journey more that just for 1 day (as I guess since you'd visit either Florence and Montepulciano), you must buy 2 separate tickets.

Hope my explanation is enough clear, Marco in Milan - Italy