Hi Mike, welcome to the RS forums -
There will be any which way to shuffle this but a couple of things to consider right off the bat?
6 different locations over two weeks are too many. You need to subtract your travel days from the total, and two nights anywhere give you only one FULL day of sightseeing once you subtract the 1/2 day you'll expend getting from one place to another. That process (packing up, checking out, getting to train station, sitting on train, getting to hotel, checking in and settled) usually takes longer than you think it will!
You definitely should fly open jaw (into one city and out of another) to maximize your time.
The very best way to get to the places you list is by train. The exception can be if exploring rural Tuscany, which can involve the need for a car. An alternative to this is basing in Florence - to avoid another physical move - and taking easy day trips to any number of places the trains/buses go. There are a few towns, like Lucca, on train lines which could work if wishing to avoid car rental but that will mean cutting something else short if this is a priority. Driving involves some homework to bone up on no-drive zones and whatnot, and I'm sure someone else will cover you off on the ins and outs.
Just as a starting place, here's a sample two-week itinerary with a Sat. departure and Sunday return. Note that I've done this by number of nights versus days as it's a more accurate method of dividing time, and dropped Milan:
S Night 1 - fly overnight (Sat) into Venice
S Night 2- Venice
M Night 3 - Venice
T Night 4 - Venice
W Night 5- Florence
T Night 6 - Florence
F Night 7 - Tuscany
S Night 8- Tuscany
S Night 9 - Tuscany
M Night 10 - CT
T Night 11- CT
W Night 12 - Rome
T Night 13 - Rome
F Night 14 - Rome
S Night 15 - Rome
Sunday - Fly home from Rome Fiumicino.
Even with Milan scrapped, this would be too many moves for us but then, we all travel differently so it may be fine for you! Still, I wouldn't give Venice any less than 3 nights - arrival day may be a jet-lagged haze - and I consider 4 nights/3.5 days a minimum for Rome. The middle could be shuffled slightly but there isn't a whole lot of margin to work with if you want to spend time in rural Tuscany. Again, keep in mind that two nights = 1.5 days of sightseeing time, three nights = 2.5 days, and so on? It all comes down to what and where you two are MOST interested in doing/seeing.
Flying into Venice and out of Rome is usually recommended as many flights from Venice depart very early in the morning.