I generally think of medium as being of some sort of the middle of a static range, (statistically it is defined as the value halfway between the mininum and maximum values), such as being of medium height, while intermediate is about the middle of a process such as the intermediate level music examinations are what you would have to pass through to get from beginner level to advanced level.
The other important point is that a given case may have several intermediate levels or states but will only have one medium point.
The other difference is that a specific sample might not contain any of the medium value but a range of states must pass through the intermediate value as they progress.
To use them in a physics sense I would say:
- in a set of samples for stress testing a medium sheer point of 10 N was found but
- in all samples 7 N was found to be an intermediate point between the elastic limit and the sheer point.
In your coupling context I would say that possibly single a system can be strongly coupled or weakly coupled and has one or more intermediate levels of coupling but if each of a set of systems can only have one level of coupling then the middle level between the highest and lowest will be medium coupling.