>>1504127>straight line would be sufficient as long as I got it centered?Most likely, yes. A good dual wallpaper won't highlight the fact that there's a physical gap between the two screens. Horizontal lines are very good for this. Sharp angled lines are very bad, when your eye follows it, the gap becomes a shit-ton more noticeable.
Unless it's something that specifically takes advantage of the gap, like this one.
But, that's just, like, my opinion man.