Obtain the complex envelope of a two-tone signal
You can represent any bandpass signal by its complex envelope.

You must, however, pick a frequency for which you do that. In your first example, that frequency is $f_c$; in your second example, you need to pick $f_1$, $f_2$ or any other frequency as $f_c$ – and then represent your cosines into products of $e^{j2\pi (f_1-f_c) t}$ and $e^{j2\pi f_c t}$.

Any textbook that introduces the concept of complex envelope (and, usually, complex baseband) should introduce that concept; if yours doesn't, it might be worth having a second textbook for cross-literature.