Good things come in pairs, and the partnership of a duet can be eternal. When in tune, one half may anticipate the other's next impulse and adjust accordingly. Then again, those pesky creative differences can split a duo's once-harmonious relationship like a chisel through sheetrock. But Bernie and Barbara McDonald wouldn't know anything about that. The pair has been playing traditional Irish music together for more than 25 years with little to bicker about. "We have a similar taste, or more like a similar vision when it comes to music," says Barbara of Bernie. "We both know, almost simultaneously, when we've done something right," Bernie adds. For more than a decade the McDonalds have played Sunday and Monday nights at John D. McGurk's (1200 Russell Boulevard, St. Louis; 314-776-8309), with Bernie singing and playing guitar and Barbara playing the wooden flute and tapping her foot in time. Though Bernie readily admits the combination of songs he and Barbara play -- ranging from his own compositions to centuries-old harp music to waltzes and jigs -- don't make up the kind of repertoire that pulls in a lot of work, McGurk's' steady stream of rowdy regulars might disagree.