For first order systems, it's easy to get the time constant by using the Thevenin concept.
Suppose I have a second-order system, for example a two-stage cascaded low-pads filter with identical resistance and capacitor values.
I am sure that this system will have more than 1 time constant.
However, my guess is that the transfer function of the desired output with respect to the input voltage will have 2 poles and the reciprocal of these poles will give the time constants.
I am confused whether depending on the output of interest the poles will change. I think they won't, but I am not able to convince myself.