Is two of something good really better than one? Rock the Garden organizers certainly hope Twin Cities music lovers think so. After three or four years of single-day tickets selling out faster than you can say, “Please tell the band Low to finish its sound check,” the 12th installment of Walker Art Center’s outdoor music fest with 89.3 the Current was turned into a two-day event for the first time this year. Because of that — and because it’s one of RTG’s more eclectic, record-nerdy lineups of late — tickets are still available to Saturday’s more festive, mixed-bag lineup with hip-hop vets De La Soul, and to Sunday’s guitar-heavy indie-rock marathon with Spoon.