There's been no sophomore slump for the McGarrigle sisters, whose self-titled first effort, "Kate and Anna McGarrigle," was perhaps the most enchanting debut album of 1976. With their second record, the newly released "Dancer with Bruised Knees," they have gone a step further, refining the blend of quietly clever lyrics and unassuming, folk-based melodies and instrumentation that made their initial outing so attractive.

Of course the McGarrigles, who will be performing with Bonnie Raitf tonight and Wednesday at the Warner Theater, weren't exactly rookies when they went into the studio to make that first record. Linda Ronstadt had already recorded Anna's "Heart Like A Wheel" and "You Tell Me That I'm Falling Down," and Maria Muldaur had done three numbers the sisters had written: Kate's "The WorkSong" and "Lying Song" and Anna's "Cool River."

But as finely crafted as these songs were, it's only with their own albums that the McGarrigles have made it clear they're not conventional singer-songwriter types. They have a much greater awareness of and respect for tradition than the average pop performer, for one thing, and they've proven remarkably skilled at fitting old-fashioned sounds and niotifs to their own sensibility and design.

In fact, listening to "Dancer with Bruised Knees" (Warner Bros. BS 3014)is a little bit like stepping into a time warp. The rich, striking harmonies, romantically melancholy melodies and spare, acoustic sound of Kate's "Southern Boys" or Anna's "Kitty Come Home" convey the impression that what one is hearing is actually a turn-of-the century parlor song, meant to be sung by a barbership quartet on a summer evening.

It's only when one stops to consider what Kate is singing on "Southern Boys" in that disarmingly gentle voice of hers that the McGarrigles' wry humor surfaces. "The breath in your car is as soft as cotton," she says of her Dixie beaus, "whether they're wooing you or whispering the latest racist joke." Teo verses later, she's telling one of her lgood old boys not to "hide it all that good old stuff down below that Mason-Dixon line."

Anna's songs, in contrast, seem to be less ironic and more sentimental. On the alblum's title tume she uses the ballet's pas de deux as a metaphor for the imbalance that has developed in a love affair, and on "Naufragee du Tendre," one of three songs sung in French, she offers a heartfelt plea not to be left "shipwrecked" while an electric piano and a horn section tinkle gently behind her.

The fondness for French comes about because Katc and Anna McGarrigle, along with sister Jane, who appears intermittently on organ on both albums, grew up in the Laurentian village of St. Sauveur des Monts, Quebec. There they acquired a taste not only for the blues, Appalachian and even Caribbean music hat has obviously influenced their style, but also for the traditional French-Canadian chanson - itself a form of folk music.

The result is "Bianche Comme La Neige" and "Perrine Etait Servante," the two lovely though thematically bizarre traditional numbers the McGarrigles perform as a seven-munute medley at the close of side one. Their arrangement is simply masterful: full, round-like harmonies and an instrumental ensemble made up of accordion, recorders, mandolin, violin, trumpet, harmonica and organ give both songs a distinctly medieval air.

Like the oddly jaunty "Lament Pour Ste. Catherine," which Anna McGarrigle and Phillippe Tatartcheff wrote for the first album, niver imagining it would end up a hit in the French-speaking section of Europe, "Blanche" and "Perrine" have a charmingly homespun character. Rather than strive for studio polish, natural sound, using friends in place of hired studio hands whenever possible, and playing piano, guitar and half a dozen other instruments themselves.

All this makes for a package so unpretentious that the McGarrigles haven't even bothered to record their best-known numbers. "Heart Like A Wheel" is the only one of their five "hits" that they've issued thus far, and that was on the last album. There's no telling exactly how many songs the McGarrigles have written or learned sonce their days in Montreal's Mountain City Four, a folk group modeled after the Weavers, but it's clearly enoughthat they can be just as pickly as they please.