One more solution..

Copyrights to my collegue sitting in next desk.. 🙂

create table t3(

n1 int not null,

n2 int not null,

n3 as case when n1< n2

then cast(n1 as varchar)+','+cast(n2 as varchar) else

cast(n2 as varchar)+','+cast(n1 as varchar) end unique

);

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

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