PreparationFor marinade: Whisk all ingredients in medium bowl to blend. For kebabs: Place fish and lamb in separate medium glass or stainless steel bowls. Add 2/3 cup marinade to each; toss to coat. Chill at least 30 minutes and up to 1 hour, stirring occasionally. Reserve remaining marinade for basting and making sauce. Drain fish. Alternate 2 fish cubes, 2 tomato halves, 2 peach wedges, and 2 onion bottoms on each of 8 skewers. Drain lamb. Alternate 2 lamb cubes, 2 bell pepper squares, 2 eggplant cubes, and 2 zucchini rounds on each of 8 skewers. (Kebabs can be assembled 2 hours ahead. Arrange on 2 rimmed baking sheets. Cover; chill.) Spray grill rack with nonstick spray, then prepare barbecue (medium-high heat). Transfer 1 cup reserved marinade to small saucepan for sauce. Grill kebabs until slightly charred and cooked as desired, turning occasionally and basting with remaining marinade, about 8 minutes for fish and 10 minutes for lamb. Transfer kebabs to platter. Bring marinade in saucepan to boil. Reduce heat to medium-low. Whisk in butter, 1 slice at a time, allowing each to melt before adding next (do not boil). Season butter sauce to taste with pepper. Serve sauce with kebabs.