The first possible place to "assassinate an assassin" is Mission 3: The Golden Cat. They appear by Slackjaw's area.
If you see one of them first, simply sneak up behind them and stab them in the back.
If one of them see you first, blink over their head and quickly stab them in the back (how I got the achievement).
The simpler way to get this achievement is to just wait until you get to the Flooded District later in the game. There are plenty of assassins everywhere, and you just have to assassinate/stealth kill one of them.