I used to practice moving my eyes independently for a long time. You can't "technically" do it unless you have a lazy eye or something because the motor controls are linked, but you can get around that too a degree by shifting your focus to extremes. For example, cross your eyes so they are both locked inwards, then look to the right, while keeping them focused close. It will apear that your left eye is stationary while your right eye moves all on its own. After a while you can pretty much move them wherever you want other than both facing outwards or one up and one down. That's still impossible