unique constraint on 2 columns

I need a certain unique constraint in pg that i can't figure out.

Given:

    create table test_a (
        id serial ,
        name_1 varchar(32) ,
        name_2 varchar(32)
    );

I need name_1 and name_2 to both be unique so that:
    name_1 never appears in name_1 or name_2
    name_2 never appears in name_2 or name_1


a standard 2 column unique index / constraint will not accomplish this.