Is there any flexibility regarding what the subject of a sentence could be, if there are two nouns in the sentence? I read that the subject is usually at the beginning of a sentence, but not always. So could I technically consider the second noun to be the subject and the rest of the sentence to be "what we know about it" / predicate?
ex. The wind blew up the chimney.
No, the grammatical subject is the grammatical subject. Words can be different parts of speech. It all depends on how a particular word functions in a given sentence.
Chimney is a noun, and what we know about it is that the wind blew up it. So can chimney be the subject and "the wind blew up" would be a fronted predicate? (this is a term we use in Arabic, I am not sure this is even a thing in English)
The worst thing a person who is trying to learn a new language can do is mix it up with the mother tongue. Comparative grammar is for scholars who want to learn ABOUT languages, not necessarily learn to use a language.
In your sentence, 'chimney' is a noun, but it is an object, not the subject.
I think the verb would be considered passive in this case.
Or is it that since the sentence is in active voice, I cannot label the sentence in a way that would require the verb to be passive?
Some ideas just don't express well in the passive and there is no need to attempt tortuous adjustments to match a grammatical idea that exist in your mother tongue.
The wind blew up the chimney.
This just cannot be expressed in the passive.
???The chimney blew up the wind. ???
We can front the adverbial phrase, 'up the chimney',
Up the chimney, the wind blew.
but that doesn't change the subject from 'wind' to 'chimney'.