To earn this achievement that's called "Put Your Heads Together", you'll have to grab two enemies at once.
This is also done on Stage 2, which is called Back Alleys on the map select screen. Start the level, then make your way to the first ladder you see and climb up to the top. To the left is a tapesmith shop, so keep going to your right until you see three Williams come out from the right side. I suggest getting close to two of them and start punching away at the both of them until they are in a stunned state, then just press B and Billy or Jimmy, whoever you are playing as, should grab both of them automatically and execute a double headbutt on each other. As he is doing that, the achievement unlocks. I did this when I was in the middle of those two stunned Williams, but I am not sure if that's for all the time or if you have to be in front of both of them. Either way, it should work, as long as both of them are stunned at the same time. I am sure, however, that this stage is the best one to get this achievement.
Have fun and good luck!