Kathy Barwick and Pete Siegfried, a new acoustic duo, will perform from 6 to 9 p.m. Saturday at the Union Square Building, 151 Mill St. in downtown Grass Valley. Cover is $5, which includes a beer or glass of wine on the house. Barwick and Siegfried combine years of experience into a compelling, highly entertaining duet sound that ranges from the Monroe Brothers and other “brother duets” of the 1930s to such iconic modern partnerships as Buddy and Julie Miller and Gillian Welch and David Rawlings. Barwick and Siegfried, aka the Mountain Laurel Duet, blend bluegrass, old-time country, traditional Irish …