Design of second order low pass filter using passive RC components and opamp
You have your sallen-key topology wrong. Even if it was correct, I don't like the circuit because attaching a load will mess with the transfer function. Ideally your load attaches to the output of the op-amp or a buffered node, as then attached loads will not draw current from the filter but the op-amp itself, preserving the filter's characteristics. 







simulate this circuit &ndash; Schematic created using CircuitLab

^ Sallen-Key two pole low-pass.



To answer your question, for the sallen-key you need to analyze as a whole (or look up TF, it's somewhat complex). If you used buffered RC/Active Lowpass, it would be each TF multiplied, as the op-amp will provide the current.