Place the yam and sweet potato in a medium pot, cover with cold water, and gently boil until just tender, not soft, about 5 minutes. Drain well, cool in ice-cold water, and then drain well again. Place the oil, citrus juices, cumin, cayenne, salt and pepper in a salad bowl and whisk to combine. Add the yam and sweet potato, raisins, walnuts and green onions and toss to combine. Cover and refrigerate until ready to serve (can be made a day in advance). Warm the salad 15 minutes at room temperature and toss gently before serving.