INSERT using results of CTE INSERT to provide unique id values
I don't understand why you need 2 tables if they have only 1-1 relationship. But here it is (pk is the primary key of t3):

WITH ins AS (
  INSERT INTO t1 (col1) 
    SELECT NULL FROM t3 
  RETURNING t1_id
) 
, r AS
( SELECT t1_id, ROW_NUMBER() OVER () AS rn
  FROM ins
) 
, t AS
( SELECT *, ROW_NUMBER() OVER () AS rn
  FROM t3
) 
INSERT INTO t2
  (col1, t1_id)
SELECT
  t.val1, r.t1_id
FROM t 
  JOIN r USING (rn) ;


If your t3 is the results of a SELECT instead of a preexisting table, you can implement it as such so that you don't have to repeat the t3 query twice:

WITH t3 AS (
  SELECT ...
), ins AS (
  INSERT INTO t1 (col1)
    SELECT NULL FROM t3
  RETURNING t1_id
), r AS (
  SELECT t1_id, ROW_NUMBER() OVER () AS rn
  FROM ins
), t AS (
  SELECT *, ROW_NUMBER() OVER () AS rn
  FROM t3
) INSERT INTO t2
  (col1, t1_id)
SELECT
  t.val1, r.t1_id
FROM t 
  JOIN r USING (rn);