Is AC widening the same as AC separation?

Not necessarily. Separation of the acromioclavicular (ac) joint is a result of an injury where the ligaments stabilizing the ac joint are injured or torn. The ac joint may be wide for reasons other than trauma such as resorption of the tip of the clavicle from conditions such as hyperparathyroidism, or chronic injury from activities such as weight-lifting.
Yes. These are essentially the same although separation may imply actual tearing of the ac ligament vs. "widening" implying a less significant problem there.