How can I correctly setup attachInterrupt() in this sketch?

[GolamMostafa]

@costalegre

I wonder if a zero crossing detection circuit would work if the oscillator frequencies were far apart. It may be better to use 2 adc inputs and compare the timing to compute the frequency difference.

If the source of interrupts for INT0 and INT1 are these sine waves, then you have to transform them into square waves to get the LOW, RISING edge, and FALLING edge.