Which of these sentences is correct: "The clock under the curtains' hour hand broke off", or "The clock under the curtains's hour hand broke off"? The actual thing being made possessive, "The clock under the curtains," is singular, suggesting that you should add 's to the entire phrase to make it possessive. This would make the latter sentence correct, but it looks funny to me. Of course you don't add 's to a plural noun already ending in "s" in order to make it possessive, but what do you do if the possessive noun phrase itself is singular but it ends in a plural noun?
The rule is that you write what you say. This is a purely phonologic law.
And you say:
The clock under the curtains’ hour hand broke off.
Therefore, that is what you write. Nobody ever ever says curtainses, so you mustn’t ever write something that says that (like curtains’s). It’s curtain’s for one and curtains’ for more than one.
In writing you can convey the difference between one curtain and several, but not in speaking. If there were just one curtain you would write:
The clock under the curtain’s hour hand broke off.
The curtain does not possess the clock. Any sentence with an apostrophe indicating possession by the curtain is wrong. Rewriting the sentence as recommended above is correct. However it is possible to have multiple possessives such as Aunt Betty's brother's car or if multiple brothers then Aunt Betty's brothers' cars.
More specific to the POp's question: you follow the same rules as other possessive.
The eye of the tiger's gaze. one eye, one tiger, one gaze
The eyes of the tiger's gaze. multiple eyes, one tiger, one gaze
The eyes of the tigers' gaze multiple eyes, multiple tigers, one gaze--as if a pack singly staring down a prey
The eyes of the tigers' gazes. multiple eyes, multiple tigers, multiple gazes
The eye of the tigers' gaze. single eye, multiple tigers, singular gaze as if the pack was staring as one
The eye of the tigers' gazes. multiple tigers, multiple gazes, but with a singular purpose