Composite field in SQL Server, does it exist?
You could leverage computed columns for this.  So your create table would look like:

CREATE TABLE foo
(ID int not null,
 OrderNumber int not null,
 OrderDate datetime not null,
 UniqueOrderNumber AS 
   (CONVERT(varchar(20),OrderNumber) + ":" + CONVERT(varchar(20),OrderDate,112))
)


SQL Fiddle Example