My two teenagers haven't spoken to each other in twelve days

My son and daughter, a year apart, have not said a word to each other in twelve days. At dinner they pass things through me. Neither will say what started it. My son says 'ask her', my daughter says 'nothing happened'. I can sit them down and insist on getting to the bottom of it, or leave them to sort it out, which is what my mother did with us. The first might make it worse. The second has not worked so far. What would you do?