This feels like an interesting interview question. I think you could simulate this with a full outer join on the entire select list and coalesce? For a UNION ALL you could put some literal column in the selects from both tables that you set to different values and include that in the join so you'd get a result set that will have all nulls in the right table columns for the rows in the left table and vice versa. Something like

  WITH top_t AS (
   SELECT
     a
    ,b
    ,c
    ,'top' as nonexistent_col
   FROM table_1
  ), bottom_t AS (
   SELECT
     a
    ,b
    ,c
    ,'bottom' as nonexistent_col
   FROM table_2
  )
  SELECT
    COALESCE(top_t.a, bottom_t.a) AS a
   ,COALESCE(top_t.b, bottom_t.b) AS b
   ,COALESCE(top_t.c, bottom_t.c) AS c
  FROM top_t FULL OUTER JOIN bottom_t 
    ON top_t.a = bottom_t.a
    AND top_t.b = bottom_t.b
    AND top_t.c = bottom_t.c
    AND top_t.nonexistent_col = bottom_t.nonexistent_col -- remove this for a normal UNION