Criminal Sentence 116: Compound Possession

Happy Halloween!

A reader, Jan, sent me this lovely sample of incorrect apostrophes, warning me that an earlier sentence indicated it was clear that Tina and John had two separate service plans:

"Janet is compiling some statistics on Tina West and John Dentins, two customer's in the database. She wants to know the date of Tina and John's first service plan order."

Obviously, there shouldn't be an apostrophe in "customer's"; it's just a plural noun. As for "Tina and John's first service plan order," Jan said it was clear that each person had a service plan, so it should be "Tina's and John's first service plan order" (or even "orders").

Compound possession means that two (or more) people are sharing something, so they share the apostrophe:

my mom and dad's house (they share a house)
my mom's and dad's houses (they each have a house)