The Hidden Blade Assassin's Creed in Depth

Anonymous Blitz
If you don't enter l'Arsenale, you will stay Anonymous and the targets won't move past their first waypoint. This is the perfect opportunity to kill them with throwing knives. They will be too far away to lock onto, so you'll have to aim "manually." Throwing knives travel in the direction of your last horizontal movement. Walk in the direction you would like to aim, and look over Ezio's head to fine tune your aim.

Double Air Assassinations
The beams (yards) on the ships give you a clear path to quickly reach the ship that the targets plan to use for escape. The archer on the ship will see you, so you'll have to eliminate him. After that, you can do whatever you please when the targets arrive on board. This ship is very similar to Marco Barbarigo's ship, so the assassinations performed from "up high" in that walkthrough video can also be done here, including the spring-loaded air assassination.
http://www.thehiddenblade.com/marco-barbarigo-assassination-variety-pack

Instead of going over water, you can take a path over rooftops that follows the two targets. Unlike other chases in the game, these two targets follow a series of fixed waypoints. Since Dante is faster than Silvio, he will wait for Silvio when he reaches one of the waypoints. This presents a nice opportunity for a double air assassination, since both targets will be near each other and stationary for a brief time.

Chasing Air Assassinations
This clip shows two places where blind air assassinations can be done on the fleeing targets. The first air assassination is on Dante. I intentionally climb to roof-level using a slower path; it delays me so that I reach the launching point of the first air assassination at nearly the proper time. I still have to slow down a bit as I approach the edge of the roof.

You have enough time to climb up to roof level again for another air assassination - this time on Silvio. When you reach roof level, watch Silvio as he rounds the corner on the way to the ship, and then start the final run to the jumping point. This air assassination is more dramatic since it is an "extended" air assassination. The range on this leap is longer than normal air assassinations since the targeting of Silvio occurs as he's running up the gangplank.

Dante, you look a little different today.
Instead of targeting Silvio, you can do this extended air assassination on Dante. Silvio follows quickly behind and stands next to Ezio as if he were Dante. It takes Silvio a few moments to notice the difference.

Two Kills, One Lock
You can lock onto a target up to a distance of 15 m. You can maintain that lock up to a distance of 30 m. Lock onto Silvio as he passes by. When he reaches the ship, he'll be nearly 30 m away. Aim your gun, but don't fire yet... There will be a brief moment when Dante walks between you and Silvio. Shoot at this moment to kill Dante. Since Silvio is still alive, your lock is maintained. Shoot again or throw a knife.

There is no spoon.
One of the reasons why developers enforce a maximum distance from the target is because the game engine needs to model the environment surrounding both Ezio and the target, which becomes more difficult as the two become more separated. The developers forgot to enforce that here. After Silvio and Dante reach the ship, you can leave the area and place the ship out of your view. Since the ship is far away and not in your line of sight, the game will stop modeling it. Suddenly faced with no ship to stand on, Silvio and Dante fall into the water and drown.

Stunt Assassination
It takes a long time to reach the jumping point on the upper wall of l'Arsenale, so you have little room for mistakes on the way there. I bump the guard off the wall to his doom, but this is unnecessary; you can just run past him instead. I'm not sure why, but I've never been able to land the stunt jump and simultaneously kill both targets with double hidden blades. Maybe you'll have better luck, or maybe there's something in the game mechanics that prevents this from happening.

Pre-assassination Assassination of Dante Moro
Before the main assassination mission begins, you need to fight Dante just outside of l'Arsenale. Throw him in the water to kill him. Due to scripting of the memory, Dante is resurrected in l'Arsenale immediately afterward.