Is Arezza really only 1/5 hour = 12 minutes south of Florence?

Pisa and Florence are close. In fact, on one point, they used the same airport. I think of Bologna as being much farther away, even though the train times might be similar. As Perche pointed out, they're really different regions and cultures, including food and wine.

My advice would be to stay in Tuscany on this short trip. Don't try to do too much and allow some time in case things go wrong.

Since the purpose of the trip is business, I think the OP should be concerned with maximizing his/her effectiveness for the business aspect of the trip and not focus on tourism and other personal activities. I would fly into Pisa, go to a hotel in Pisa, and spend the remainder of the arrival day gently exploring Pisa a bit, but do not attempt to do everything. In the summer, there will be lines and crowds. After the meeting, go to Arezzo, check into a hotel, and again relax and gently explore that city a bit. After the meeting ends the next day, you could go to Florence (can you stay over the weekend to extend the business trip at your own expense?), but it would certainly be safer to stay close to the airport before a morning flight.