Is there a reliable way to double assassinate?

I'm playing Assassin's Creed: Brotherhood but I assume this applies to AC2 as well.

I'm trying to do an especially difficult Borgia captain assassination and it's proving frustrating because double assassinations seem to be very hit-and-miss. Sometimes I only assassinate one instead of both. Is there any surefire way to do it?

I don't think there is a surefire way of doing this, however you can improve your chances with the following things.

1. Position yourself in the middle of the guards, and make sure you are not locked onto any of them, and not looking at either of them either. [ this make that particular guard a target ]
2. Have the double blades equipped
3. Hold R1 [ assuming ps3 ], to perform the aggressive take down.
4. Throw down a smoke bomb to make it exceptionally easy.
5. Points 1-4 apply to air double assassinations as well, however I would suggest trying it from a ledge where you are close enough to the guards [ close enough that you can actually lock on to one of them ], so that you don't go into leap of faith.