Achieving "sequence number", rather than dateTime, for updates; postgres specifically
As a_horse_with_no_name originally suggested in question comments:

--assuming we have tables: a, b, ...
CREATE TABLE a
(
  a_id integer NOT NULL PRIMARY KEY,
  a_name text
) ;

CREATE TABLE b
(
  b_id integer NOT NULL PRIMARY KEY,
  b_name text
) ;


First we create a global sequence and add a column in each table:

-- create one global sequence to be used by all tables
CREATE SEQUENCE global_sequence_number ;

-- add a column in each table, that uses the same sequence
ALTER TABLE a
  ADD COLUMN global_sequence_id INT NOT NULL 
    DEFAULT nextval('global_sequence_number'::regclass) ;

ALTER TABLE b
  ADD COLUMN global_sequence_id INT NOT NULL 
    DEFAULT nextval('global_sequence_number'::regclass) ;


Create trigger function (one) and UPDATE triggers (one in each table):    

-- create a trigger function to update the column
CREATE OR REPLACE FUNCTION update_global_sequence_id()
RETURNS TRIGGER AS 
$$
BEGIN
   NEW.global_sequence_id = nextval('global_sequence_number'::regclass); 
   RETURN NEW;
END;
$$ language 'plpgsql';

-- add a trigger, in each table
CREATE TRIGGER update_a_global_sequence_id
  BEFORE UPDATE ON a FOR EACH ROW 
    EXECUTE PROCEDURE update_global_sequence_id();

CREATE TRIGGER update_b_global_sequence_id
  BEFORE UPDATE ON b FOR EACH ROW 
    EXECUTE PROCEDURE  update_global_sequence_id();


And profit:

 -- example of use
INSERT INTO a (a_id, a_name)
VALUES 
  (1, 'a'),  (2, 'b') ;

INSERT INTO b (b_id, b_name)
VALUES 
  (3, 'c'),  (4, 'd'),  (5, 'e') ;

SELECT *
FROM a FULL JOIN b ON FALSE 
ORDER BY COALESCE(a.global_sequence_id, b.global_sequence_id) ;



-- output
  1   a  1
  2   b  2 
             3   c  3
             4   d  4
             5   e  5


And after some updates, to check that the triggers work:

-- lets do some updates:
UPDATE a SET a_name = 'aa' WHERE a_id = 1 ;
UPDATE b SET b_name = 'cc' WHERE b_id = 3 ;

-- and see what happened
SELECT *
FROM a FULL JOIN b ON FALSE 
ORDER BY COALESCE(a.global_sequence_id, b.global_sequence_id) ;



-- output
  2   b  2 
             4   d  4
             5   e  5
  1  aa  6                   -- sequence updated to 6
             3  cc  7        -- sequence updated to 7