You could ungroup your weapons (there should be an option when your right click on your guns in space) Also you can stack them by simply drag & drop them over each other.
But there is a simple reason you wouldn’t like to do it:
The faster you kill the NPCs the less time they have to deal damage to you. Focus fire on one target until it is dead is usually better than to split fire.
If you want to stop shooting, you can either klick your weapons or on the little icon which is displayed underneath the target you are currently shooting at.