Like Per already mentioned you need to separate the two "informations" in the frequency domain if you only have one line for the transmission.

Using higher FSK frequencies can be problematic though, as the resulting bandwidth can go beyond the capabilities of line. An example: The FSK frequencies 1300Hz and 2100Hz are often used. What counts here is not the absolute frequencies but their ratio. In order to be able to distinguish them in the receiver the freqencies should be as separate as possible. On the other hand, many demodulators are sensitive to the harmonics of FSK frequencies, so a compromise must be found which is the reason for this frequency ratio. If you use much higher FSK frequencies, well beyond the analog frequencies also to be transmitted, they must also fullfill this ratio, which can result in a too wide bandwidth only for the FSK.

Another approach is to use a carrier well above the audio frequencies and modulate the frequency of this carrier by the FSK frequencies. Here again 1300Hz and 2100Hz can be used resulting in a much lower overall bandwidth. A LM331 could be used for modulation and demodulation as a start of experimenting.

How to choose the right carrier frequency and how to filter the audio and modulated carrier (from unwanetd side bands) is a different story, though. Also, your entire equipment must be linear enough so that no intermodulation occurs injecting intermodulation frequencies right into the audio band.

Kai