Hitting down the line for one handed is usually easier than two handed. The tendency for a two handed backswing is to go cross court. Where as one handed have to do more to hit cross court.
You have to remember to not try to hit on a rope, shortening your swing, but to trust yourself and know your swing width is roughly the same as your height.
To practice hitting down the line on a two handed backswing, turn your body and show your shoulder to your target.
To hit cross court on a one handed backswing, open up your body and be early and bring your back foot through.