A dual crush is done by standing next to one other player and pressing the RT button when it appears on the screen.
You need to make sure you have at least between 60 and 70 MP for each character, Sometimes I needed 60 others a bit more.
I simply got it against the first boss. Got in a CO-op two player session and just beat him up until he jumps and breaks the floor, falling to the bottom.
Then kill all his cronnies down the bottom, then you and your buddie perform either 2/3 dual crushes and once it dies. Ping goes the acheivement.
Note: - You get different dual crushes with different combination of characters. Some appear to be stronger than others. Personally I prefer the dual Alucard one.