1. You have a warm heart.

2. You have warm hearts.

3. You two have a warm heart.
4. You two have warm hearts.
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Does the subject in Sentence 1 singular?
Does the subject in Sentence 2 plural?
Is #3 grammatical? What about #4?

'Yes' to your first two questions.

4 is far better than 3 since it's rare for two people to have one heart between them!!