There's a difference in purpose between using two wrenches and using a keyed washer. You heed two wrenches to properly hold the upper cone while tightening the locknut, or turning both against each other if you prefer. That's a matter of getting it tight.

The keyed washers job has nothing to do with getting it tight. It's job it to prevent the two parts on either side of it from turning together and working loose as a pair. Keyed washers were standard in headsets and axles for decades, but are often omitted these days allowing parts to go out of adjustment even when tightened very well.