How do 2 people without sickle-cell anemia to have a child with sickle-cell disease?

Yes. The defect of sickle cell anemia occurs when 2 sickle cell genes are passed to a newborn. Since each parent passes half their genes at conception & it takes 2 sickle genes to cause disease, one gene comes from each parent. Many parents carry this gene quietly, unknown to them because it doesn't make a single gene carrier sick.
It's required. A child with sickle cell disease inherited a sickle gene from both parents, therefore both parents must carry the gene. Millions of people in the world have sickle cell trait (they carry one normal gene and one sickle gene). They are generally healthy. But, if two of them have children together each child has a 25% chance of inheriting sickle cell disease by inheriting the sickle gene from both.