A splitter is an enemy who physically splits into two enemies or more, creating a clone of itself. It basically multiplies. What you need to do is kill two at once with one shot. The first time you see this enemy is the perfect time to get the achievement. When going for the item near the train tracks during the "Find items for rewriting reality" objective, this enemy will jump out from the door of a building you are supposed to enter. The game will inform you of this new enemy. Shine your light on it and to make it easier, let it split again. Now you will have 4 enemies to shoot at. Take your shots. If your achievement doesn't unlock, just reload your last checkpoint. It will put you right in front of the building again. You can always get it later, but it's nice to get it out of the way here.
Go to Act 3: Completed. If you have 40 manuscripts completed you'll be able to get the Combat Shotgun. It's near the Main Observatory. Once you collect it go over to the doors. 2 Splitters will pop up. Shine your flashlight at them. Once you see that 2 Splitters are close together fire and the achievement should pop up.
The easiest way to do this is get a Splitter on his own. use a flare to make him split twice. so their is 8 of him. As soon as you drop the flare run a quarter circle to either side. Use a shotgun, everyone is saying combat shotgun, but I used the good old pump action one. Mainly cause it's not auto and you don't fire 2 shots at the same time the quarter turn should put 2 or more of the little splitters in a line the flare works best on them if you can lure the splitter into a smaller area. Hope this helps everyone Later DK :)
I got it on the final drive-in level, as I was running around to the back of the power station. You're jumped by a splitter in a very tight space, so when they split they don't have much room to run. So with the combat shotgun it's easy to take out two in one blast.