Do you understand the difference in these two sentences?
Sam and Celeste’s problems are not my concern.
Sam’s and Celeste’s problems are not my concern.
In the first sentence, Sam and Celeste share problems. In the second sentence, Sam and Celeste each have their own problems. A single apostrophe signifies shared possession, whereas individual apostrophes signify separated possession.
This is why you wouldn’t say, “Sam and Celeste’s new haircuts look great.” They can’t share haircuts, so it should correctly read, “Sam’s and Celeste’s new haircuts look great.”
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Timothy