Hi,
I've been to Tuscany more than a few times, some of them with bike, though never with tandem.
Yes it is hilly. As far as I could tell it's hard to cycle anywhere without going over a hill, plus a lot of the interesting villages are built on hills so you've got to get up them.
The hills are steeper than many alpine climbs, with 8-12% and more for short sections is common. Obviously the hills aren't as long as alpine climbs, with most about 1 mile or so. If you can practise this sort of riding before going you'll have more fun. If you don't have hills like this a ride up, a multistorey car park (after hours) could be good training ground.
In terms of gearing I have 39x25 on my single bike and had to grind up more hills than I would have liked. A compact crank on the single bike would have been advisable unless you're racing fit and want to race up all the hills. A tandem triple will be fine. Ours has ridiculously low 30x34 and we have never used it.
Road surfaces are pretty variable, just like the UK, so I'd recommend 28mm tyres on the tandem. Again lots of the interesting places to visit have gravel / unmetalled drives so it's not the place for tandeming on 23mm racing tyres.
Lastly don't blindly trust the signposts. I remember going for a 20 mile ride and following signs to St.Margerita part way through the ride. After a big hill and a loooong steep descent I found myself in an unfamiliar valley by a river with the daylight running out. After looking at a bus stop to find out where I was I phoned my wife who picked up in the car (without any directional issues as she's much better at that sort of thing than I am). On getting home I looked at the map and found there are no fewer than 5 St. Margeritas in within 10 miles. Things are just a bit different...
Hope that helps. I would also talk to the folks at Santana as it's their tour and they probably know everything there is to know about the route.