This question is about use of the group genitive versus joint possessives in a phrase and/or list.
Where two or more distinct persons, animals, etc., are in the genitive, the group genitive applies only when there is joint possession, responsibility, relationship, as in 'William and Mary's reign' and 'Jack, Tom, and Mary's uncle.' If two separate possessions or other relationships are concerned, each noun must clearly be shown in the genitive.
(Eric Partridge, You Have a Point There, Routledge, 1978)
In this case there is the example of a grammar question on a Jetstream (ESL) test. There are two possible questions, A: "What are Liam's and Noel's jobs?" versus B: "What are Liam and Noel's jobs?" (Answer: They're singers.)
Jetstream claims that 'B' is the correct question, i.e. group genitive. Why would 'A' NOT be correct given that 'jobs' is plural and Liam and Noel each have their own respective jobs? Is this because 'singers' here acts as a type of job and they both share the same type of job? Or could Jetstream be wrong on this one? Or is it an ambiguous case?
Absent the answer, one might not presume when asking the question that both Liam and Noel have the same type of job, so could it still be correct to use the joint possessive form?
The test simply had the following list without possessive forms and the student had to supply the apostrophes and s's where appropriate. The italicized answer in parenthesis was the only correction the system made and it specifically indicated that "What are Liam's and Noel's jobs?" was incorrect.
Is this your umbrella? – No, it isn't. It's my friend's umbrella.
Are these your keys? – No, they aren't. They're Peter's.
What are Liam's and Noel's jobs? (What are Liam and Noel's jobs?) – They're singers.
Is your husband's name Sam? – Yes, it is.
What are your daughters' names? – Their names are Tracy and Lily.
Is this the teacher's computer? – Yes, it is.
That's a nice phone! – Yes. It's Colin's. It's the new Samsung.
When is your parents' anniversary? – It's on November 17th.
