I'm sure this is trendy because of the Canal+ series Borgia(recommend for any renaissance fans), but in the show Alessandro Farnese and Cesare are close friends, studying together in Pisa for priesthood. I can't find any evidence directly proving this, but it doesn't seem unlikely seeing as they would both have been in Pisa at similar times, they were close in age, and Alessandro's sister was the mistress of Cesare's father. If anyone has anything to prove or disprove this, please let me know. :)

Also, if you read that and don't have an answer, I would also be interested in any documentaries or books about historical figures from 1000-1700 Europe(I find it much easier to take in information when it is presented through the scope of politics surrounding an important individual as opposed to a description of events).