The writer speaks of two people keeping each other warm on a cold night. Alternate translation: "If two people lie down together at night, they can be warm" (See: [[rc://en/ta/man/translate/figs-explicit]])
This refers to a person lying down. The author use this rhetorical question to emphasize that two people can keep each other warm but one person cannot. This question can be written as a statement. Alternate translation: "a person cannot be warm when he is alone." or "a person who lies down alone cannot be warm." (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-ellipsis]])