Yep, and while one of them sings, get the other one to watch out for certain techniques (breathing, neck tension etc.). Keeps the other one involved and means they get used to correcting each other away from your lessons.
Most def – it’s not a method or a quick-fix, more like a scientific description of what’s actually going on. It’s the basis of what I’ve taught ever since I took one. And they’re comparatively cheap! Do it!