Split screen currently not being planned doesn't have anything to do with competitive questing. Most people would play cooperatively, and just because the other person can see your side of the screen, doesn't mean they will be following it closely enough to catch any sabotage attempts before they happen (in some cases you can literally just choose the dialogue for your quest, or attack an NPC, etc).
Competitive questing was a main new feature during the Kickstarter, and split screen was still mentioned then, conceived of as 2 players in split screen being able to play online with 2 others in split screen, or 1 or 2 others playing separately.
Check back when controller support is implemented, and see if there is any change/news on split screen feasibility.
