In a 6-qt. stockpot, heat butter and oil over medium heat. Add onions; cook and stir 6-8 minutes or until tender. Add garlic; cook 1 minute longer., Add squash and broth; bring to a boil. Reduce heat; simmer, uncovered, 10-12 minutes or until squash is tender. Stir in 1 can of beans, parsley, tarragon and salt. Puree soup using an immersion blender. Or cool soup slightly and puree in batches in a blender; return to pan. Stir in remaining can of beans and heat through. Top each serving with yogurt.