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As far as I know - it is really a problem to find suitable answers.
According to my experience we can say the following:
* A multi-loop feedback system has more than one single feedback loop (that`s logical).
In most cases, we can identify one or more internal loops and one "outer" overall loop.
In this case, it seems to be the most logical method to use the outer loop only for stability analyses because this is the loop with the largetst time constant
Hence, all internal loops remain closed and the outer loop has to be opened for loop gain analyses.
* There are certainly cases where it is not possible to discriminate between inner and outer loops.
In this case, we have - step by step - to investigate the loop gain (and the corresponding phase margins) of each single loop (all other remain closed).
I think, it makes sense to say: The loop with the smallest stability margin determines the stability properties of the system.
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