Say we're talking about contraception, and the probability of one contraceptive, $A$, working is $99\%$, and the other, $B$ is also $99\%$. What is the probability of them working using both at the same time?
When I tried raising 99% to the power of 2, I realized I only got the effectiveness rate of contraception used at two separate events. Other similar operations haven't worked.
So I'm not sure what else to do here.
Edit: I'm aware medical statistics are extremely complicated, so I'm only assuming independent conditions for conventional purposes to get some idea of what the probability looks like.