If the outcomes of two risky events are independent and their probabilities of causing death are p1 and p2, then the probability of at least one of them leading to death is p1 + p2 - p1*p1. So, using micromorbs m1 = p1*10^6 and m2 = p2*10^6 leads to the following micromorb value for two independent risky events

m1 + m2 - m1*m2*10^-6