Viator does not run any tours, as it is only a booking agency with a poor reputation for after-sales service. Siena will be very busy the day before the Palio, and 2 hours is very little time to see this amazing city.
I second sydneynick's excellent suggestion and add one I received in these forums, which turned out to be great advice - take an early bus, and have breakfast at Nannini on Banchi di Sopra. It's an awesome place to soak up the local atmosphere and I imagine it would be even more so the day before Palio.
Definitely want to do a tour - thoughts?
If you're really set on a guided tour, then at least look for one that lasts more than a couple of hours, and rather than booking through viator, book directly with the tour company. Googling it suggests a tour just of Siena is hard to find, most are like the one you mentioned above. Two hours is not long, and when a place is crowded, as Siena will be the day before the Palio, it's even less. You will end up seeing little and being constantly worried about "missing the bus"
why do you 'definitely want to do a tour'? The day before Palio will be crazy...Siena will be packed and the city's stimulation on overload. Read up a little about Siena...take it all in.
Chester9cat is absolutely right.
Don't even think of visiting Siena during any events associated with the two Palios -- unless you know somebody who owns one of the palaces on the Campo.
I love Siena, but during Palio the town is a complete madhouse and one of the worst tourist traps in the world.