This means that one of them is 1 and the other is greater than or equal to 1. We can write this as:
$P(Y=1) = P(X_1=1, X_2 \geq 1) + P(X_1 \geq 1, X_2=1) - P(X_1=1, X_2=1)$
Now, we can use the probability mass function of the Poisson distribution to find these
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