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If you're trying to write about possession and you have two subjects, you have to decide if the two people possess something together or separately. Here's an example: Steve and Amy's religious beliefs.
The rule is if the two people share something, you use one apostrophe s. So if Steve and Amy have the same religious beliefs, it is correct to say Steve and Amy's beliefs with only one apostrophe s after the last noun.
On the other hand, if Steve and Amy have different beliefs, then you would say Steve's and Amy's beliefs.
The rule is that if each person “possesses” something different, then you use two apostrophe s's.
The quick and dirty tip for remembering the rule is to think about luggage and hairdryers. Imagine that two women are going on the same trip. If they are sharing an adventure, they could share a hairdryer on the trip, so then they can share the apostrophe s (Amy and Mignon's adventure); but if they are each going on their own separate adventure, then they each need their own hairdryer, and they each need their own apostrophe s (Amy's and Mignon's adventures). So an apostrophe s is like a hairdryer: You don't need to bring two if you are going to stay in the same hotel room.
There you go. That was a short excerpt from Grammar Girl's Quick and Dirty Tips to Clean Up Your Writing. I think one reason many people don't know the answer to that question is that they don't know that it's called compound possession when there are two people involved**. Therefore, it’s hard to look up the answer because you don't know what to search for. I hope my answer helped.
* Capitol. is capitalized when referring to the United States Capitol building in Washington, D.C.
** Compound possession is also called joint possession