Yes (unless you're Vin and hack the system.) If both Mistborn are burning Atium, then each person will throw off multiple, ever changing Atium shadows. If one Mistborn avoids all of the other's Atium shadows, then he will never be hit by the person himself. The atium shows all possible futures, which means that the one that will happen is still shown. it is difficult to avoid all of them yes, but with enough skill/speed, mental or physical, (preferably both) it is possible to beat atium as long as you also have atium.