Solution (source code)

= Solution

Put $s=\lambda+\mu$. Solving either Kolmogorov equation gives
$$
P(t)=\frac1s
\begin{pmatrix}
\lambda+\mu e^{-st}&\mu(1-e^{-st})\\
\lambda(1-e^{-st})&\mu+\lambda e^{-st}
\end{pmatrix}.
$$
These entries have the correct initial values and their rows sum to one.