what is the Input impedance of transmission line, when source frequency contains two frequency components?
If your system is linear, you can analyze the behavior at each frequency seperately. You don't need to define a single input impedance that applies to both frequencies. You analyze the response to stimulus at \$f_1\$ using the input impedance at \$f_1\$, and you analyze the response to stimulus at \$f_2\$ using the input impedance at \$f_2\$. 

Then you use the linearity of the system so that the response \$y\$ to the combined stimulus \$x_1+x_2\$ is just \$y(x_1+x_2) = y(x_1) + y(x_2)\$