Solo uno activo para cada restricción de relación

Tipo de

Cupon - is bound to -> Order;
Order - has active -> Cupon;

Cupon (
Id PK,
orderId FK Order.Id,
Unique ( Id, orderId) -- any superset  of PK is unique
);

Order (
Id PK
ActiveCuponId,
(Id, ActiveCuponId) FK Cupon( OrderId, Id)
);

Ver violín https://dbfiddle.uk/?rdbms=sqlserver_2017&fiddle=596b30905d02a9be5ab5c5aff