As far as I know, you never hear them singing a note in any other song that is not backing up a note sung by Maines, who has a great, strong voice. For good reason, I guess.
Not to nitpick, but there are TWO types of "good singers". One is the Natalie Maines, belt it out (although she's too nasal) lead singer, and the other is the kind of singer that has such control over their pitch they can sing in perfect harmony every time. I do believe the sisters fall under that category.
I'm a little biased, because I am primarily a harmony singer.