[little sage]

Happy Camper,
I did a search and found part of your answer.

When two people jointly possess one thing, you can use one appostrophe:
"Tim and Jenny's car"

Use apostrophes twice when the people possess things separately:
"Tim's and Jenny's cars"

If one word is a possessive adjective, then both words should be possessive, so
"It's my and Jenny's car"
but I think "mine and Jenny's car" is wrong...

I'll keep looking for a better explanation...