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Add -'s to each noun in individual possession.
Example: Olga's and Joanne's books are valuable.

Add -'s to only the last noun in joint or group possession.
Example: Anne Smith and Glen Smith's article on solar heating interests me.

http://www.cameron.edu/~carolynk/plural_nouns.html

1. Carl's and my cat is outside. (Grammar Plus, Level One, page 29)

2. Carl and my cat is outside.

3. Carl's and my cats are outside.

Are the red parts in the above sentences group possession or individual possession?
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You can tell whether they are talking of one or more cats from the use of 'is' and 'are'.

If it is one cat then this is a case of group possession - they share a single cat.

If it is more than one cat it is not clear; they might own the cats separately or jointly. We can't tell tell from this sentence.
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It is obviously intended to be joint possession-- there is only one cat.

With more than one beast, however-- Carl's and my cats-- it is confusing. Quirk is right in calling this coordination of genitive + possessive pronoun awkward, no matter how you do it: Carl's and my cats? My and Carl's cats? Carl's cats and mine? Only the last makes it reasonably clear that each individually owns more than one cat.

PS: Sorry Nona-- didn't see you slip past.