MIDDLEBURY — When Bill Sims Jr. and Mark LaVoie perform at Middlebury College and the Art House in the Marble Works this Thursday and Friday, it won’t be their first time playing together. Since Sims, a guitarist and vocalist from New York City, first met Bristol native and harmonica virtuoso LaVoie at a blues festival in Burlington back in 1992, their mutual interest in folk music and the blues has blossomed into a 17-year friendship and collaboration.