For first order systems, its easy to get the time constant by using thevenin concept.
Suppose i have a second order system ,for example consider a two stage cascaded LOW PASS FILTER with identical resistance and capacitor values.
I am sure that this system will have more than 1 time constant.
My guess is that the transfer function of desired output with respect to 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, will the poles change, i think it won't but i am not able to convince myself.