I'm slightly messed up with the Clausius statement of the 2nd Law.
I've seen at least two versions, which seem to be conceptually different.
a) It is impossible to transfer heat from a colder body to a hotter body without any other effect.
b) It is impossible to transfer heat from a cold (thermal) reservoir to a hot (thermal) reservoir without any other effect.
I would like to use the Clausius statement to rule out heat transfer from a colder body to a hotter body, where both have finite thermal capacity. No work put in. If we suppose this transfer could happen, then the hotter body would become hotter and the colder body would became colder. So, there actually is some other effect (temperature changes). How does a) rule out this procces? Obviously b) does the job.