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@BruceET the link is broken. Nevertheless, I found this. But I'm still doing something wrong. If $T = min\{t_1,t_2\}$ and $t$ is the measured time, therefore: $P(T>t)=P(T_1>t,T_2>t)=(1-F_1(t))(1-F_2(t))=e^{-(\lambda_1-\lambda_2)t}$. It should be $1-e^{-(\lambda_1+\lambda_2)t}$