"Helen and Kathy's reactions."

or

"Helen's and Kathy's reaction."

quote:

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To indicate individual ownership, add the apostrohe and s to each name.
Al's and Sue's cars [note that cars is plural]

To indicate joint ownership, add the apostrophe and s only to the last name or to each name.
Al and Sue's car OR Al's and Sue's car

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quote:

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I was typoing away

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