We can look at a more general case that I believe will help solve this problem.
\begin{eqnarray*} P(X|X+Y=n) &=& \frac{P(X=x, Y=n-x)}{P(Z=n)}\\ &=& \frac{P(X=x)P(Y=n-x)}{P(Z=n)} \\ &=& {n \choose x} \left( \frac{\lambda_1}{\lambda_1+\lambda_2} \right)^x \left( \frac{\lambda_1}{\lambda_1+\lambda_2} \right)^{n-x} \end{eqnarray*}
Which is a binomial pmf with $p = \left( \frac{\lambda_1}{\lambda_1+\lambda_2} \right)$ and expected value $E(X|X+Y=n) = np = n \left( \frac{\lambda_1}{\lambda_1+\lambda_2} \right)$