These two statements are semantically the same.

You probably wanted to write this instead:

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

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

Another possible solution to this requirement might be ("might" because I don't know the business problem):

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)

);