uniona_all_diff_schemas.sql

WITH 

T_A AS (
  SELECT STRUCT("1"  AS x,  2 AS y) a -- schema a is huge
),

T_B AS(
  SELECT STRUCT(TRUE  AS z,  4.0 AS w) b -- schema b is huge as well
)

SELECT 
  a,
  NULL b
FROM
  T_A
  
UNION ALL

SELECT
  NULL a,
  b
FROM
  T_B
  
  
# Wanted output:  
#  (1 x, 2 y) a,            NULL b
#  NULL       a, (TRUE z, 4,0 w) b