Chris,

The buisness requirement is

n1,n2 shd be unique even when interchanged.

unique constraint on 2 columns shd work even when values are interchged.

n1 n2

11 10 ->allowed.

10 11 -> fail as 11,10( 10,11's already exists

your example i am afraid will fail as n1 > n2 at first insert itself, however by req it shd be allowed.