Siegecraft Commander

Timing is important.

two player multiplayer setup (local)
-build an outpost with each player in range to kill the other
-fire one shot each, and this should bring each tower to half health
-fire the second shot each with timing such that one player shoots just before the other one

This results in one tower destroying another, but the shot will get off to destroy the original tower. Achievement pops.

If not done via splitscreen this is actually fairly easy on the third level right at the beginning where you see the outpost and ballista right next to each other, just shoot the outpost once, and then shoot once more after the enemy outpost shoots.

I also made a video for those of you who like to see it done instead.
(3:48)