This is better done when you go to the oil well in order to rewrite reality in the first chapter of the game.
First, you will need the shotgun. You can find it in a building that is located in front of the motel and a little to the left. It is easy to spot, because it is near a car with its lights on and a Arizona Petroleum sign. The shotgun is in the front steps.
Once you obtained the shotgun and collected all three required objects (Valve, battery, CD), go to the well. When you arrive, you'll see a cutscene where 2 splitters come out of the hole.
Simply aim at them with your flashlight. Each will divide in two. Aim to each of that four, and they will become eight. So, now, it's very easy to align and kill two of those eight enemies with a single shotgun shot.
EDIT: Also, there's another simple way: if you enconter a single Splitter when carrying a shotgun, make it divide once. Now, aim with your flashlight to one of the new two guys. When it starts dividing, shoot. You should kill both divisions in one shot.