public class Customer
{
    public int CustomerId { get; set;}
    public int CustomerName { get; set; }
}

public class Order
{
    public int OrderId { get; set; }
    public int CustomerId { get; set; }       //BuyerCustomer
    public int CustomerSecondId { get; set; } //ReceiverCustomer
    public Customer BuyerCustomer { get; set; }
    public Customer ReceiverCustomer { get; set; }
}

SELECT a.*, b.*, c.* FROM dbo.PRODUCTS_ORDER a
INNER JOIN  dbo.CUSTOMER b ON a.CustomerId=b.CustomerId
INNER JOIN   dbo.CUSTOMER c ON a.ShippingCustomerId=b.CustomerId

List<Order> order= null;
 order= (List<Order>)
                        var con=connection.Query<Order, Customer, Customer, Order>(sql,
                                (order, customer1,customer2) =>
                                {
                                    order.BuyerCustomer = customer1;
                                    order.ReceiverCustomer = customer2;
                                    return order;
                                }, splitOn: "CustomerId,CustomerSecondId ");