Usage of 's on genitive case!

I would use "Joe and Mary's backpacks."

John and Mary’s marriage took place yesterday. John is now a slave to Mary.
Mary’s and Betty’s marriages took place yesterday. Now each of them has their own slave.



John and Mary’s rooms were 202 and 203. They both used 202 and 203 together.
John’s and Mary’s rooms were 202 and 203. John used 202 and Mary used 203.

Nice and funny response.Now, I clearly understand when one becomes slave.:-).

lol, thankyou, excellent examples; I don't think I'm forgetting that any soon.

AnonymousHow do I use it correctly when there are two subjects?

Example:

Joe and Mary's marriage.
OR
Joe's and Mary's marriage.
(if they are married to each other)

Joe and Mary's backpacks.
OR
Joe's and Mary's backpacks.
(each has his/her own backpack).

Thanks.