double person possessive

A lot has been written on possessives. But I dont think anyone asked about this:

a. Tom and Sarah both share one car. So we say:

1. It is Tom and Sara's car.   OR

2. It is Tom's and Sara's car.

3. What if (unlike the above ) each has their private car

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Lots of people query this. Generally the answer is that if you two are seen as a single unit, it is usual to restrict the 's to the second name: It's Mom and Dad's anniversary tomorrow.

With reference to #3 = They are Tom's and Sara's cars.

Hi,
I was told both are used and are ok.

So, if there is a car that belongs to both Tom and Sara,
Tom and Sara's car.
Tom's and Sara's car.
Tom's car and Sara's.
Saras's car and Tom's.

If there is a car that belongs to Sara and a car that belongs to Tom,
Tom's and Sara's cars
Tom's car and Sara's.
Saras's car and Tom's.

If there are some cars that belong to both Tom and Sara,
Tom and Sara's cars.
Tom's and Sara's cars.
Tom's cars and Sara's
Sara's cars and Tom's.

If there are some cars that belongs Tom and some cars that belong to Sara,
Tom's and Sara's cars.
Tom's cars and Sara's
Sara's cars and Tom's.

They are ambiguous of course, you need context to understand. I hope I didn't make any typos. What do you think of my choices? (non prescriptive choices )