Hi,
If the frequencies are both the same then there's no way to 'add' them without a non linear network.
Two 12MHz sine signals do add, but the result is another 12MHz sine signal of different amplitude or zero depending on phase.
To get twice the frequency a number of different methods are used. One is a tuned collector. Another is to use a bridge rectifier followed by a filter. The bridge rectifier converts the sine into a full wave signal which contains the 2nd harmonic. Then it's just a matter of filtering the other harmonics down low enough so that they dont matter to the output signal.