This time around you have 2 targets to go after, Dante and Silvio. Simultaneously. And I do mean simulataneously.
When you reach the final showdown mission, you will meet up with Dante first with Silvio taunting from behind somewhere, but concentrate on wiping out the guards (and protecting the general as needed) as no matter how you whale on Dante, he won't go down or be wounded apparently. (That's my experience at least. If you guys find a way to assassinate them this early post up!)
It's only when the cutscene with the boat appears and Silvio and Dante retreat that you will able to do anything with them.
Run after the two blaggards. Guards will blockade you along the way but just assassinate one of them (those jump kills are a rush!) to clear a path and move on. Don't worry about the timer too much. Concentrate on chasing them down.
Silvio seems to be the slower and won't put up any resistance. I took him from behind.
Dante will use his weapons on you, however, and here's where being clever might be better. I had chased the two to the boat so with Dante aboard, I grabbed him and threw him off into the water.
If you'd noticed, even though your character can swim now, all others (even friends, sadly) cannot. That's essentially a one-hit kill on the brute known as Dante.
Revel in your victory and go on to the mystery of their planned trip...
TRIVIA: Notice you got a letter out of the kills. I found it romantic that Dante's wife tried to reconnect with her love. Makes me wonder what Dante truly thought. ;)