Toss the asparagus with 1-1/2 tablespoons olive oil and season with salt and pepper. Grill, covered and turning once, until tender and grill marks appear, about 9 minutes. Meanwhile, rub the remaining 1-1/2 tablespoons olive oil on the salmon and season with salt and pepper. Place the salmon on the grill, cover and cook, turning once, for 6 minutes for medium-rare. Transfer the asparagus and salmon to a platter and drizzle with the balsamic butter.