Q. When using a pronoun to replace the first noun when two nouns show possession of one item, which case should the pronoun be? For example, in the sentence “I’m going to my uncle and aunt’s house,” “uncle” is not in the possessive case. So which case should the pronoun be? “I’m going to him and my aunt’s house”? “I’m going to he and my aunt’s house”? Or, “I’m going to his and my aunt’s house”? And, if the answer is “his,” how do you reconcile that the pronoun is not agreeing with the noun it replaces in gender, number, and case? And what is correct if the pronoun replaces the second noun? “Megan’s and his room”? Or “Megan and his room”?

A. The trick of showing joint possession with a single apostrophe s is possible only with two items that can take an apostrophe s. Hence a car owned by John and Jim can be expressed as “John and Jim’s car.” This is clearly a convenient shorthand—helped out by the fact that, normally, you can assume readers will not think that you’re writing about John, on one hand, and Jim’s car, on the other. Most pronouns do not form the possessive with an apostrophe s. “One” becomes “one’s,” but “he” becomes “his” and “I” becomes “my.” Therefore you generally cannot use shortcuts in cases of joint possession involving a pronoun. You must make both owners possessive:

his and my aunt’s house
Megan’s and his room

You can reconcile the first as equivalent to “my uncle and aunt’s house” by remembering that the apostrophe s after “aunt” also applies to “uncle”; “his” is technically replacing “uncle’s” not “uncle.”

There is quite a lot of discussion of this point online. All those who claim grammatical expertise agree with Tony. When there is more than one possessor, all of them must be in the possessive form. The exception is that when both (or all) the possessive forms end in 's, it is permissible and customary to omit all but the last 's. So instead of
- Mike's and Sally's house
you tend to say
- Mike and Sally's house.
"Mike" here is an elliptical form of "Mike's". This is the key to the following Q&A from the Chicago Manual of Style Online, which goes on to address the point we are dealing with here:


"Q. When using a pronoun to replace the first noun when two nouns show possession of one item, which case should the pronoun be? For example, in the sentence “I’m going to my uncle and aunt’s house,” “uncle” is not in the possessive case. So which case should the pronoun be? “I’m going to him and my aunt’s house”? “I’m going to he and my aunt’s house”? Or, “I’m going to his and my aunt’s house”? And, if the answer is “his,” how do you reconcile that the pronoun is not agreeing with the noun it replaces in gender, number, and case? And what is correct if the pronoun replaces the second noun? “Megan’s and his room”? Or “Megan and his room”?

A. The trick of showing joint possession with a single apostrophe s is possible only with two items that can take an apostrophe s. Hence a car owned by John and Jim can be expressed as “John and Jim’s car.” This is clearly a convenient shorthand—helped out by the fact that, normally, you can assume readers will not think that you’re writing about John, on one hand, and Jim’s car, on the other. Most pronouns do not form the possessive with an apostrophe s. “One” becomes “one’s,” but “he” becomes “his” and “I” becomes “my.” Therefore you generally cannot use shortcuts in cases of joint possession involving a pronoun. You must make both owners possessive:

his and my aunt’s house
Megan’s and his room

You can reconcile the first as equivalent to “my uncle and aunt’s house” by remembering that the apostrophe s after “aunt” also applies to “uncle”; “his” is technically replacing “uncle’s” not “uncle.”"
http://www.chicagomanualofstyle.org/qanda/data/faq/topics/Po...


So the requirement that all the possessors should be in the possessive form also applies to pronouns; but since their possessive forms do not end in 's, the option of omitting 's from all but the last does not apply. Therefore it should be "your and Sally's house", not "you and Sally's house". You can't say "you house" instead of "your house", so you shouldn't say "you and Sally's house" instead of "your and Sally's house".
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And yet people do, very commonly. Judging from Google results—that rerum omnium arbiter of our times—"you and your wife's X" and "your and your wife's X" are just about equally common. It is clear that the latter, which normative grammar clearly mandates, is preferred in careful and formal writing (and speech), but the former seems natural to many native speakers. Here’s a wordreference.com query on Spanish-English translation of a phrase which means "your and your wife's full names". A native speaker comments:

"Although some might quibble over the grammar, many native speakers would say the following (and I would agree with them):
I'm going to need the following information: you and your wife's full names, just as they appear on your passports....
http://forum.wordreference.com/showthread.php?t=2230124&lang...

Examples like the following are common:

" When both you and your wife's names are on the mortgage, you're both legally obligated to pay the debt regardless of any other circumstances."
http://everydaylife.globalpost.com/wifes-name-off-mortgage-1...

It seems clear to me that whatever normative grammar may say, native speakers tend to find it awkward to use a possessive form in joint possessive expressions for any but the last possessor. They find it awkward to say "Mike's and Sally's house", and reduce it to "Mike and Sally's house". The usage is well established, and normative grammar approves. Even if there is a real possibility of misunderstanding, as in "Mike and Sally's uncle" (one person or two?), I think most people would prefer to rephrase it, or just to say "Mike and Sally's uncle" and trust that people will understand what they mean, than to say "Mike's and Sally's uncle".

When we have pronoun plus noun, as in the example before us, "your and Sally's" is correct, but somehow awkward. It's not the kind of thing many people would come out with in informal speech. You can rephrase it, if there's a satisfactory way of doing so that doesn't change the meaning significantly and is not too cumbersome (this is by no means always the case), or you can simply reduce "your" to "you", which is what many people do. I would actually say that most people do it in normal speech. Indeed, resistance to "your and X's Y" is so strong that you can actually find a case like the following, on a law firm's website:

" You may wait one to two years for your trial, but meanwhile, both you, and your wife’s attorneys’ fees will continue to climb."
http://cordellcordell.com/practice-areas/divorce/

That comma after "you" makes it quite comical when read with a grammarian's eye, and of course no grammarian would recommend it, but I think we have to recognise that, in practice, people don't handle joint possession the way normative grammar says they should.

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Note added at 15 hrs (2014-02-23 11:36:47 GMT)
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It ocurred to me that the tendency to omit the possessive marker from possessives before the last might reflect an unconscious the idea that "Mike and Sally" or "you and Sally" are really a single joint possessor: it is the house of Mike-and-Sally, or of you-and-Sally, so Mike-and-Sally's house or you-and-Sally's house. However, that explanation wouldn't hold for "you and your wife names", which means your name plus your wife's name and is not a case of a single joint possessor.

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Note added at 15 hrs (2014-02-23 11:37:03 GMT)
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Sorry: occurred, not ocurred.

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Note added at 15 hrs (2014-02-23 11:37:35 GMT)
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And "an unconscious idea", not "an unconscious the idea".

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Note added at 15 hrs (2014-02-23 11:42:51 GMT)
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What lies behind these reflections is that although there are such things as agreed prescriptive rules of grammar which need to applied for certain purposes, grammar is not confined to them. Those who study the English language ought to pay attention to what native speakers actually say. Diachronic linguistics teaches us that usages people prefer tend to win out in the long run and turn into the prescriptive consensus.