This is perhaps more of an American English question, but I'm not sure.
I'm Swedish, so...

I'm messing around with a Python¹ tool to produce "correctly" formatted
film titles. As I have read² a film title should take into consideration
what kind of word it is before that word is capitalized.

¹the programming language
²somewhere online, can't find that site, but I'm sure a search should
reveal the info

Example:

1. This Is a Film Title beyond Belief

whereas:

2. This Is Not A Film Title Beyond Belief

The words capitalized in the #1. sentence are not members of the word
groups of articles, prepositions or conjunctions.

Now, the difficulty lies in conjunctions which consist of two words, they
are (from what I have found, perhaps there are more?) two:

3. not only

4. but also

The Title "rules" also said, that one should make sure that the first and
the last words are always capitalized, even if they should happen to be
members of the aforementioned groups of words.

Now, here's my conundrum: If the text begins with a double-word
conjunction, how is it to be capitalized?

Like this:

5. Not only This but also That

or like this:

6. Not Only This but also That

That is, should the conjunction, which consists of two words, have both
words capitalized (the two are treated as one), or should only the first
word be capitalized? Because neither of the word "only", nor the word
"also" belong in the groups of exception words. See at the bottom for a
list of those words I have found.

Yes, Python has a builtin function to use, but it only creates the version
found in the #2. text, above.

Not asking for an absolute rule here (unless there really is one, or you
have a good explanation as to why), more like a majority wins
situation. :)

Thanks.

Addendum, exception word list:

# articles
'a', 'an', 'the',
# prepositions
'above', 'about', 'across', 'against', 'along', 'among', 'around', 'at',
'before', 'behind', 'below', 'beneath', 'beside', 'between', 'beyond',
'by', 'down', 'during', 'except', 'for', 'from', 'in', 'inside', 'into',
'like', 'near', 'of', 'off', 'on', 'since', 'to', 'toward', 'through',
'under', 'until', 'up', 'upon', 'with', 'within',
# conjunctions
'either', 'neither', 'nor', 'whether', 'after', 'although', 'as',
'because', 'before', 'if', 'once', 'since', 'than', 'that', 'though',
'till', 'until', 'when', 'where', 'while', 'and', 'but', 'or', 'for',
'so', 'yet', 'not only', 'but also'

--
for(int i=0;1<99;i++) {printf("meh");}