Based on your somewhat incomplete travel itin and trip background, I assume train is your main mode of transportion, and you only have 1 day during which you want to cover Siena (and possibly Pisa).

I think you would save time and energy if you train it from Rome to Siena, then do Florence. Be worth while to spend a night in Siena (most people would spend more time in Siena and might use it as base to see Tuscany. I would not advise trying to do Pisa and Siena in 1 day.

If you provide more details of your trip plan, FTers might be able to further assist you.