1) In a serving bowl, combine the prepared kidney beans, chickpeas, onion, celery, cucumber, parsley and dill (or mint).
2) Make the lemon dressing: In a small bowl, whisk together the olive oil, lemon juice, garlic, salt, and pepper flakes until emulsified. Pour dressing over the bean and vegetable mixture and toss thoroughly.
3) Serve immediately for the most flavor, or let it marinate in the refrigerator, covered, for a couple of hours or longer. Leftovers should keep well, covered and refrigerated, for several days. If necessary, wake up leftovers with a little sprinkle of salt or drizzle of lemon juice.