Had they both sung at the same time the sound projected would be more powerful, however because one is slightly off time, the power is lost and the lyrics become muddied.
Truth is: Most "stereo widening" plugins are essentially just cheating.
They take the single source, duplicate the same source digitally, and then time shift it by 12-24 milliseconds. The result is a "fake" stereo width. It does sound real though in your headphones... and that's where the problem lies.
It seems so quick and easy to obtain that sound you want, so you start applying it all over the mix. Then, the moment comes when you fold it down to Mono or you put it in the car and... yuck.
It's a mess! Tomorrow, I'm going to show you how to address the issue and gain a better understanding of true width!
Stay tuned!
Some of this was addressed in the recent mix I did here, or down below, if you want to see/hear it in action.