Answer:
The two girls' parents were also close friends.
Step-by-step explanation:
In general, there are two ways to create possessive nouns, that is, nouns that show ownership:
1. By adding an apostrophe (') and a s at the end of nouns that do not end with the letter s and then adding the thing that is being owned, for example, the girl's toy, the children's guitars, the cat's bed (As you can see, it does not matter if the nouns are plural or singular).
2. By adding only an apostrophe (') at the end of nouns that end with the letter s and then adding the thing that is being owned, for example, the girls' toys, the dogs' food, the cats' bed. This is the rule we must apply to the sentence: The two girls' parents were also close friends.