A frequency difference of 1 percent is probably close enough.
From B to C on the musical scale is 6 percent. That's wide enough to be distinguished by a narrow bandpass filter.
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Two stages of RC low-pass filters are sufficient to shape a square wave into sine-like. (Frequency of dtmf rows is 697 to 941 Hz.)
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The lissajous figure compares the final waveform to the original sine. Notice it is almost circular, meaning they almost match. The sharp corners of the incoming square wave are filtered out. If desired, add another RC stage, although it would attenuate the signal further.
Only the RC networks are needed once you have a bipolar AC square wave. I guess your MCU makes 5v DC pulses, so you might want a DC blocking capacitor, then 1 or 2 or 3 RC filter stages.
For the dtmf columns (1209-1477 Hz), make a similar filter, but with reduced C values. Attach it to the other MCU output.