A second order low pass filter (LPF) can be made by cascading two active LPF (an RC network + opamp.) What happens if, instead of immediately cascaded, an active LPF acts at the input (buffer) of a circuit and another one at the output of the same circuit? Say, the input and output of another intermediate audio circuit. Would the 40dB slope of a second order filter also be attained, or does the effect of a second order LPF (40dB cutoff slope) only result if two first order filters are immediately cascaded?
Would the 40dB slope of a 2nd order filter also be attained?
Overall yes but, for the intermediate circuit between the two filters, only 20 dB per decade slope would be present at its input. This part of my answer also assumes that the intermediate circuit does not introduce any extra filter slopes i.e. it is a flat-gain amplifier for example.
Or the effect of a 2nd order LPF (40dB cut-off slope) only results if two 1st order filters are immediately cascaded?
No, 40 dB per decade can be obtained this way providing the intermediate circuit doesn't introduce another slope.