Combine two SQL columns into one, selecting distinct results

example.sql

CREATE TABLE _test_1 (
    a varchar(64));

CREATE TABLE _test_2 (
    b varchar(64));

INSERT INTO _test_1 (a)
    VALUES ('john'),
           ('sue'),
           ('bob'),
           ('john'),
           ('sally'),
           ('kevin'),
           ('alex'),
           ('john'),
           ('kevin');

INSERT INTO _test_2 (b)
    VALUES ('john'),
           ('mike'),
           ('alex'),
           ('eric'),
           ('john'),
           ('carla'),
           ('carla');

SELECT DISTINCT ON (t1.a, t2.b) coalesce(t1.a, t2.b) AS out
    FROM _test_1 t1
    FULL JOIN _test_2 t2 ON t1.a = t2.b;

DROP TABLE _test_2;
DROP TABLE _test_1;

Output is:

  out  
-------
 alex
 bob
 john
 kevin
 sally
 sue
 carla
 eric
 mike
(9 rows)