Etymology of English suffix -logy

The answer to your question is twofold:

1) These are not really separate meanings in the sense of 'treat' and 'speak' being separate words. Just because two different 'senses' are listed in a dictionary, it does not mean that there are two separate meanings (like 'bank') in the minds of the speakers. You could easily see how 'speaking' about a certain subject in a certain way could be viewed as 'treating' it and the difference simply emerging from context.

2) There are many ways through which words acquire additional senses, sometimes even splitting into pure homonyms. They often involve things like metaphoric extension (as with logos), metonymy (as with 'foot' of the mountain or supposedly 'bank'), or some other figurative process. Frequently, the new senses become primary (as with 'awesome' or 'mistress'). Things are even more complicated when these developments happen across languages through poor learning or simply new contexts. New contexts also play a role in historical developments within a single language (as with 'sir').