FSK analysis: only ever find one frequency
You can make a very simple frequency discriminator by multiplying your received signal with a delayed version of itself, followed by a low pass filter. Adjust the delay to set the frequency range of your discriminator. This works because a multiplier followed by a low pass filter is a phase detector (multiply two sinusoids with same frequency and different phase and you will get the sum and the difference of the frequencies: the sum is the doubled frequency to filter out with the low pass filter and the difference is the phase). A change in phase versus a change in time is frequency, therefore the low pass filtered multiplication of the signal with a delayed version of itself will be proportional to the frequency over that particular interval. Choose your low pass filter and delay based on your symbol rate.

Optionally you can make a tuned discriminator with two pass band filters with your signal fed into the input of each one. The BW of the filter depends on your symbol rate. Follow each filter with a magnitude detector, and subtract one detector output from the other. The output of the subtractor will be your demodulated FSK.