Using small sharp knife cut off peel and white pith from 1 grapefruit. Cut grapefruit crosswise into 4 slices. Cut second grapefruit in half and squeeze enough juice to measure 3 tablespoons. Whisk juice, olive oil, and next 6 ingredients in small bowl. Season dressing to taste with salt and pepper. Toss greens with 2 tablespoons dressing in medium bowl and divide between 2 plates. Arrange grapefruit slices and avocado alongside and spoon remaining dressing over.