-- Adding constraint to an existing table
ALTER TABLE SomeTable WITH CHECK ADD CONSTRAINT [UK_SomeTable_SomeCompositeKey] UNIQUE( KeyColumn1, KeyColumn2 )

-- Adding constraint when creating new table
CREATE TABLE SomeTable
(
	KeyColumn1 INT NOT NULL,
	KeyColumn2 INT NOT NULL,
	CONSTRAINT [UK_SomeTable_SomeCompositeKey] UNIQUE
	(
		KeyColumn1, 
		KeyColumn2
	)
)

insert into t3(n1,n2) values (10,11);
insert into t3(n1,n2) values (11,10);

CREATE TABLE dbo.t3 (
     n1 int NOT NULL
    ,n2 int NOT NULL
    ,CONSTRAINT UQ_t3 UNIQUE (n1, n2)
    ,CONSTRAINT CK_t3_n1_LE_n2 CHECK (n1<=n2)
);