I wouldn't recommend doing that, as the tour still ends up in Florence and then you'd need to get to back to Siena. I'd search for a tour the starts in Siena, to ensure you return by the end of the day. There isn't a full day one like that, but there are plenty others here: http://www.partner.viator.com/en/8357/travel-tips/Tours-in-Siena-and-from-Siena/ttd
The climb up the Leaning Tower is not too hot, as there are lots of small windows and openings along the way and then you come out on a first level which is actually the the outside... then more stairs. So it depends on the day for sure!