The method I used to unlock this achievement was getting in conflict with a lot of guards at once, but using blunt-force/non-lethal weapons to incapacitate them all rather than kill/stab them. Examples of weapons that can be used to do this are hammers, maces, and rifles (not shooting the rifles, but using them as a blunt object; I personally did it this way as I had a high level rifle that was a 3-hit incapacitation for even high-level guards, assuming they didn't parry me.)
Once everyone has been incapacitated with your blunt-force/non-lethal weapon, the people you just fought should all be on the ground rolling around in pain. You will notice that when you're standing over them, you have the option to Assassinate them with
. Stand over one of them while your co-op partner stands over another one, count down, and press
at the same time. Doing so will stab the incapacitated guard with your hidden blade, killing him, which will count as a double assassination assuming your co-op partner did the same.
If you fought a big group of guards (which is pretty common,) it's very easy to rinse and repeat this process with all of the incapacitated guards to get around 5 co-op sync kills in just a few seconds. I felt this way requires a lot less coordination with your partner since the guards you're killing aren't moving around or actively fighting you and you won't trigger any further conflict doing this, since all of the guards in the area you were just engaged in are incapacitated and lying on the ground.