I had always read that if you could only have one light, you should put it on the bars.
Since I have started night riding, I would have to disagree. I want light pointed at where I am looking and on the helmet is the only way to get that.
Forget about whether two allows you to see better than one. I believe in redundancy in case one goes out.
For safety, you can tell the difference between a car coming at you with only one headlight working versus two. You can certainly be seen better by cars if you have two lights.
Aim the bar light at the right edge of the road so that you are able to see critters that may be waiting to jump out at your. Motorcyclists that have supplemental headlights generally aim the right one that direction.